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Relative sizes of the Planets. 



OUTLINE 




OF 



A S T B N M Y 



By C. LIST, 



ON THE BASIS OF THE ELEVENTH LONDON EDITION 
OF THE TREATISE 

By the Rev. T. G. HALL, M.A., 

PUBLISHED UNDER THE DIRECTION OF 

THE COMMITTEE OF GENERAL LITERATURE AND EDUCATION, 

APPOINTED BY THE SOCIETY FOR PROMOTING 

CHRISTIAN KNOWLEDGE. 



PHILADELPHIA: 
THOMAS, COWPERTHWATT & CO. 

AND 

CAREY & HART. 
1850. 



Qhv* 



Entered, according to Act of Congress, in the year 1846, by 

Thomas, Cowperthwait & Co., 

in the office of the clerk of the district court of the United 
States, in and for the eastern district of Pennsylvania. 



B. MEARS, STERLOTYJ'ER. 



i4) 



CONTENTS. 



Preface Page vii 

Section I. Introduction 9 

II. Astronomical Terms .... 13 

III. The Figure and Magnitude of the Earth 25 

IV. Motion of the Earth round its Axis and in 

its Orbit 28 

V. The Sun 34 

VI. Day and Night.— The Seasons . . 39 

VII. Atmospheric Refraction .... 48 

VIII. The Moon 56 

IX. The Phases of the Moon ... 70 

X. Tides 80 

XI. The Solar System 84 

XII. Mercury . . . . . . .88 

XIII. Venus 91 

XIV. The Earth 93 

XV. Mars .... . 95 

XVI. The Asteroids 98 

XVII. Jupiter 102 

XVIII. Saturn 105 

(5) 



6 CONTENTS. 

XIX. Uranus 108 

XX. Comets . Ill 

XXI. Meteors 118 

XXII. Zodiacal Light 121 

XXIII. The Fixed Stars 124 

XXIV. Nebula* 129 

XXV. Time and its Divisions .... 142 

Glossary 147 



PREFACE. 



Though no branch of study will fail to produce 
good results if properly pursued, yet some are of 
much more general utility, and much more appro- 
priate for persons in every condition of life, than 
others. On this ground, at least, may a high rank 
be claimed for Astronomy. There may be many 
pursuits to which it is not directly applicable, and 
many of the duties of life may be performed with- 
out a knowledge of it. But there is no one, how- 
ever favoured his station may be, or however 
humble, who would not derive enjoyment from the 
consideration of the beauties of this science, re- 
finement and intellectual discipline from the study 
of its harmonies, and elevation of sentiment from 
the contemplation of those stupendous works of 
the creative hand which it unfolds to our view. 

It may be suitable for many of the inhabitants 
of other countries, whose physical resources are 
not only limited from smallness of territory, but 
permanently appropriated by comparatively few 
individuals, to devote themselves to the Languages, 

(7) 



Vlll PREFACE. 

Metaphysics, and other branches whose cultiva- 
tion requires only leisure and a library. But in 
this country, where equality of rights, and almost 
boundless regions, with every variety of natural 
productions, tempt all to enterprise and the exer- 
cise of skill, the study of the sciences is not only 
peculiarly advantageous, but absolutely necessary 
to those who do not wish to fall behind their co- 
temporaries, in their industrial pursuits or mental 
accomplishments. 

With these views the following work is offered 
to the American youth of both sexes. 

It was the aim of the compiler to present a work 
which should be within the reach and comprehen- 
sion of all ; and which, while it served as an in- 
troduction for beginners, should communicate a 
general knowledge of the science to those who 
may not have an opportunity for its farther prose- 
cution. 

The English work, which was employed as a 
basis for this volume, is very small, and did not 
seem altogether adapted in its style to the Ameri- 
can learner. By far the larger portion of this 
treatise is therefore original. ? 



SECTION I. 
INTRODUCTION. 

The sky is the most magnificent object that is 
presented to human vision ; whether viewed by 
day, when the sun rolls over it and illumines its 
whole extent with a flood of light, or at night, 
when it is studded with a thousand stars. It there- 
fore naturally attracts our attention. 

But if, in a cloudless night, we closely observe 
its appearances, we find them constantly varying. 
We see star after star rising in the east, while 
others, with equal regularity, are setting in the 
west. We find that some in the north remain 
permanently in view, while others in the south 
scarcely rise above the horizon before they again 
disappear. We find that those which are seen at 
one period of the year in the evening, are seen at 
another period in the morning — while some which 
shine brightly at one time, are totally invisible at 
another. And, finally, that now and then one 
comes in view where none was seen before, and 
after remaining for a while, disappears, apparently 
for ever. 

(0) 



10 OUTLINES OF ASTRONOMY. 

Such being the nature of the sky, it is not won- 
derful that men early endeavored to record its 
appearances and explain its changes ; and these 
endeavors gave rise to the science which we call 
Astronomy. 

What nation first cultivated this science cannot 
now be decided. The Chinese, Indians, Chaldeans, 
and Egyptians, claim to have been acquainted 
with it thousands of years before the Christian 
era ; but their records are neither authentic nor 
instructive. The Chinese mingled it with politics, 
the other three with religion — and they all applied 
it to Astrology, or the superstitious prediction of 
events by reference to the stars. This prevented 
investigation, and consequently progress in the sci- 
ence, and gave those classes to whom its cultivation 
was intrusted, an interest in perverting it. 

The Greeks also, among the ancient nations, 
early cultivated astronomy. As with them it had 
no reference to government, religion, or astrology, 
it flourished much more than it had among the 
older nations. Thales foretold an eclipse which 
happened in the year 610 before Christ. Yet, for 
centuries after this period, their astronomy was 
so imaginary, and so mingled with their mytho- 
logy, as to be of very little utility. It was not till 
the time of Hipparchus, about B. C. 160, that it 
became a practical system, founded on observation. 



INTRODUCTION. 



11 



In the early ages of our era, the science was 
cultivated with some diligence by the astronomers 
of the celebrated school of philosophers instituted 
at Alexandria, in Egypt, among whom Ptolemy 
was the most eminent. 

For the preservation of astronomy during the 
middle ages, and the few advances which were 
made in it during that period, we are indebted 
almost entirely to the Arabians and Persians. 

Notwithstanding the many ages which had 
elapsed since it had first engaged the attention of 
the most learned men, the science was still com- 
paratively little advanced at the commencement 
of the fifteenth century. But since then it has 
been studied in every Christian country. It has 
received contributions from hundreds of eminent 
men ; but especially from Copernicus, Tycho 
Brahe, Galileo, Kepler, Newton, and Herschel. 

QUESTIONS. 

What is the most magnificent object which we can be- 
hold ? What changes do we perceive in the sky if we at- 
tentively observe it ? 

What gave rise to the science of Astronomy ? 

What nations claim to have been acquainted with as- 
tronomy long before the Christian era ? What is said of 
their records ? To what did these nations apply the sci- 
ence ? What is Astrology ? What effect had these mis- 
applications ? 

What other nation early cultivated this science ? Why 



12 OUTLINES OF ASTRONOMY. 

did it flourish much more among them than among the older 
nations ? Who foretold an eclipse ? When did it happen ? 
Why was their astronomy of but little utility for several 
centuries after this period ? When did it first become a 
practical system ? 

By whom was astronomy cultivated in the early ages of 
our era ? Who was most eminent among them ? 

To what nations are we indebted for the preservation of 
astronomy during the middle ages ? 

In what state was the science in the beginning of the 
fifteenth century ? What is true in regard to it since then ? 
From what eminent men has it received contributions ? 



SECTION II. 

ASTRONOMICAL TERMS. 

Astronomy, like every other science, has terms 
which are peculiar to it, because there are things 
to be expressed in it which do not occur in any- 
other. Of these we shall explain the most im- 
portant. The learner should endeavor to under- 
stand them clearly. 




View of the Earth in space. 

If we could see the earth from a distance, we 
should see that it is a vast globe rolling in space, 
2 (13) 



14 OUTLINES OF ASTRONOMY. 

like the moon, and completely surrounded by 
stars. 

But as we stand upon the earth, and as it is so 
large that its surface seems level to us, we can at 
any one time see but half of the sky. At the first 
glance of the observer, all the stars appear to be 
at an equal distance from him, and seem to be 
placed on the hollow surface of a vast bowl, or, as 
it is properly called, a concave hemisphere, of 
which he occupies the centre. 

The circle which bounds his view where the 
earth and the sky seem to meet, is called the Ho- 
rizon. The level on which he stands is named the 
plane of the sensible horizon, to distinguish it from 
the plane of the rational horizon, or that which 
passes through the earth's centre. They are al- 
ways parallel to each other. The sensible hori- 
zon of course varies according to the elevation of 
the observer, while the rational horizon is unvary- 
ing. 

If the spectator directs his attention to the north, 
he will find that some stars, as the Pole Star, and 
the stars of the Great Bear, never reach the hori- 
zon, but continue to move round a fixed point in 
the sky, which, from that circumstance, is called 
the Pole. The pole star is quite near the point 
referred to, and may be distinguished by observing 



ASTRONOMICAL TERMS. 15 




the direction of a line drawn through two of the 
stars of the Great Bear, called the Pointers. 

If the spectator, whom we may at first suppose 
to be placed on the northern half of the earth or 
hemisphere, be now removed to the southern hemi- 
sphere, similar appearances will present them- 
selves. The stars will rise in the east, and, after 
teaching their highest point, will set in the west. 
Some, however, will never rise or set, but revolve 



16 OUTLINES OF ASTRONOMY. 

round a fixed point, the Pole of the southern 
hemisphere. 

But, although a change takes place in the ap- 
pearance of the heavens at each instant, almost all 
the stars retain their distance from each other, and 
hence are called Fixed Stars. All the fixed stars 
describe circles round one of the points which we 
have mentioned, and which are therefore called 
the poles of the world. 

The pole elevated above our horizon is the 
North Pole ; the opposite one, which is situated 
beneath the horizon, is the South Pole. An ima- 
ginary line, joining these two points, is called the 
Axis of the world. 

But while by far the greatest number of stars 
are fixed, that is, retain their distance from each 
other, some few have motions of their own, and, 
after the lapse of considerable intervals, will be 
found to have moved among the stars which were 
in their neighborhood at the time of the first ob- 
servation. These are called Planets, or, as the 
name signifies in Greek, wandering stars. 

Among the many interesting questions which a 
youthful observer would be disposed to ask, while 
contemplating the sky, the following will probably 
occur : What becomes of the stars during the 
day ? Do the heavens undergo the same changes 



ASTRONOMICAL TERMS. 



17 



then as at night ? The important discovery of the 
telescope has enabled us to answer these questions, 
and by its assistance the stars can be seen at any 
hour of the day, and at any elevation above the 
horizon. Hence it appears that the stars are hid 
from us, not because they cease to shine, but be- 
cause their light is effaced by the more vivid 
splendor of the sun. 

It has been stated that the stars revolve round 
two fixed points, called the poles — the one being 
in the northern, the other in the southern part of 
the sky. This may be illustrated as follows : Let 
s e n w represent the horizon, o the place of the 
spectator in the centre, s and n the south and north 
points, and e and w the east and west points of 
the horizon. r 



F/ V> 

f o 1 

b\ /a 



Then, if a star rise at a, it will, after ascending 
to a certain height above the horizon, descend and 
2 * 



18 OUTLINES OF ASTRONOM5T. 

set at point e, which is at the same distance from 
the west that a is from the east. 

The place at which it reaches its highest point 
in the heavens, is in a circle that passes directly 
over head, and through the north and south poles. 
This circle is called the Meridian. 

Meridian is derived from two words signifying 
mid-day, because at twelve o'clock the sun is 
always in this circle. Now if the spectator turns 
to the north he will find that if a star rises at d, 
it will, after reaching the meridian, descend to f, 
which is at the same distance from w that d is 
from e. Hence all the stars revolve round the 
two points s and n. 

Since the meridian passes over the head of the 
spectator, it is different at different places ; thus 
we hear of the meridian of Greenwich, of Paris, 
of Washington, &c, and hence it cannot be twelve 
o'clock at the same time, except at those places 
which lie under the same meridian— twelve o'clock 
being the hour at which the sun is in the meridian. 
To find the place of the meridian, or to draw a 
meridian line on the earth's surface, is one of the 
most important objects that can engage the atten- 
tion of the astronomer. 

The following simple method may be employed 
when no great accuracy is required : — 



ASTRONOMICAL TERMS. 



19 



^ 


1 


«/ S C 


\ \* 




— ^ x\ 


111 A i J 1 



Upon a flat surface of stone or wood, as f d, 
let a number of circles be described round a com- 
mon centre ; at this centre, which we may sup- 
pose a, let a thin piece of metal, or a needle, be 
fixed upright, and let the stone be placed in a per- 
fectly horizontal position, and in the rays of the 
sun. Before noon, observe when the shadow of 
the needle just touches the circumference of one 
of the circles, and mark the point ; as the time 
approaches noon, the shadow decreases, and the 
point of the shadow will successively be found in 
the different circles. After noon the shadows 
lengthen, but are thrown in a contrary direction ; 
thus if a a represent a shadow before noon, a b 
will be a shadow after noon. Join b a ; divide it 
into two equal parts at c. Join a c : a c pro- 
duced is the meridian line M. The object of the 
numerous circles is to correct the error which 



20 OUTLINES OF ASTRONOMY. 

might be made if we relied on the chance of a 
single observation. 

The twelve o'clock hour line, in the common 
dials, coincides with the meridian line, and hence 
it is, that when the sun is in the meridian, the 
shadow of the style of the dial falls upon the twelve 
o'clock hour line. 

The Sensible horizon has been defined to be the 
plane on which the spectator stands ; and the 
Rational horizon to be the plane which passes 
through the earth's centre, parallel to the former. 
The rational horizon is of course an imaginary 
one. Astronomers, in treating of the fixed stars, 
imagine themselves to be standing on the plane 
of the rational horizon. This substitution, though 
the one horizon is nearly four thousand miles from 
the other, is not attended with any inconvenience ; 
for, when compared with the vast distance of the 
fixed stars, the whole earth dwindles into a mere 
point. 

The Zenith is the point in the heavens immedi- 
ately over the head of the spectator, and therefore 
through this point the meridian passes. 

The Nadir is a point also in the heavens, but 
directly below his feet, and consequently opposite 
to the zenith. 

The Equator is a circle, every point of which 
is at an equal distance from both poles, dividing 



ASTRONOMICAL TERMS. 21 

the earth into two equal parts, one of which is called 
the Northern, the other the Southern Hemisphere. 

This circle, extended to the fixed stars, is called 
the Celestial Equator. 

The Ecliptic is the apparent path of the sun 
among the fixed stars, which cuts the celestial 
equator at two points, called the Equinoctial Points, 
or the Equinoxes. 

These circles may be represented by two hoops 
of the same magnitude, the one crossing the other, 
as in the figure ; the two points of section e and 
f are the equinoxes. 




The Latitude of a place on the earth is its dis- 
tance from the equator, measured on a circle pass- 
ing through both poles. 

The latitude of a heavenly body is its distance 
from the ecliptic. 

The Altitude of a heavenly body is its distance 
above the horizon, measured on a circle passing 
hrough the body and the zenith. 



22 OUTLINES OF ASTRONOMY. 

The distance of a star from the celestial equa- 
tor is called its Declination ; this distance is mea- 
sured on a circle passing through both poles. 

Two heavenly bodies are said to be in Conjunc- 
tion when they are on the same side of the earth, 
and to be in Opposition when the earth lies exactly 
between them. 






(1) 
(2) 



M 

-+- 

E 



M 



Thus if s, m, and e represent the sun, moon, and 
earth, figure 1 represents the moon in conjunction 
with the sun, and figure 2 in opposition with it. 

These terms will be readily understood by at- 
tending to the following diagram. 




The smaller circle is intended to represent the 
earth. 



ASTRONOMICAL TERMS. 23 

a is the place of the spectator. 

z is his zenith, z the nadir. 

n and n the north and south poles of the earth 

e a the earth's equator. 

p and p are the poles of the heavens. 

The line p p the celestial axis, e q the celestial 
equator. 

d a h the sensible, k Jc the rational horizon. 

a e the latitude of the spectator at a. 

s l is the declination of a star at s, and h s is 
its altitude or distance above the horizon. 

n a e 7z, or the circle through a n and w, is the 
terrestrial meridian. 

p z p q is the celestial meridian, or the circle in 
which the stars are highest above the horizon. 

The learner will find it very useful to his pro- 
gress to make himself well acquainted with this 
figure ; and it will be interesting to him in pro- 
portion to his familiarity with it. 

QUESTIONS. 

What has astronomy in common with every other sci- 
ence ? Why has it these ? 

If we could see the earth from a distance, what should we 
discover it to be ? How much of the sky can we see at 
one time ? What is the appearance of the stars and the 
sky ? Do all parts of the concave appear equally distant 
from the observer ? 

What is the horizon ? What is the level on which 



24 OUTLINES OP ASTRONOMY. 

the observer stands called? What is the plane of the 
rational horizon? How are the sensible and rational hori- 
zons placed in regard to each other ? How does the first 
vary ? 

Where may we see stars that never reach the horizon ? 
What are some of these stars called ? What is the point 
round which they move called ? How may the pole star be 
found ? 

If the spectator were transferred to the southern hemi- 
sphere, what would he find there ? Round what point do 
the stars revolve there ? Why are nearly all the stars 
called fixed stars ? 

What is the axis of the world ? What two points does 
it join ? 

Though most of the stars are fixed, what is true of 
others ? What are they called ? What does their name 
signify in Greek ? 

What questions are likely to occur when we view the 
sky ? What has enabled us to answer these questions ? 
Why are the stars hid by day ? 

What is the meridian ? Why is it so called ? At what 
places only can it be twelve o'clock at one time ? Where 
is the sun at twelve o'clock ? 

What is the distance between the sensible and the ra- 
tional horizon? Why may one be substituted for the 
other ? 

What is the zenith ? What is the nadir ? What is the 
equator? Into what two parts does it divide the earth? 
W T hat is the celestial equator ? What is the ecliptic ? At 
what points does it cut the celestial equator? What is 
the latitude of a place ? What is the latitude of a hea- 
venly body ? What is altitude ? What is the declination 
of a star ? What is meant by conjunction? What by op- 
position ? 



SECTION III. 

THE FIGURE AND MAGNITUDE OF THE 
EARTH. 

A spectator at sea, or in a level country, may 
imagine himself in a flat plane, extending itself in 
all directions from him, and bounded by the circle 
of the horizon, which separates the sky from the 
sea or land. 

But a little attention will soon convince him that 
the surface even of the sea, in a dead calm, is not 
perfectly level, but that it is a curved surface, 
convex towards the heavens — that is, shaped like 
the outside of a globe, and that he occupies its 
highest point. 




For, at sea, the first part of a distant ship that 

is visible is the top of her masts ; then, more and 

more of her masts is seen, and gradually the hull. 

On land, also, we observe the tops of distant ob- 

3 (25) 



26 OUTLINES OF ASTRONOMY. 

jects, when the lower parts are invisible, as we 
may particularly notice in the case of lofty mount- 
ains. This cannot be otherwise explained than 
by a curvature of the earth's surface. Above all, 
the voyages of modern navigators have clearly 
proved this fact ; for by continually sailing either 
eastward or westward, they again arrive at the 
port from which they set out. 

Thus the earth is an opaque body, of a globular 
form, but not a perfect globe, for it is somewhat 
flattened at the poles, and bulging at the equator. 

The magnitude of the earth is next to be con- 
sidered, by which is meant its circumference and 
diameter or thickness. 

Every circle may be conceived to be divided 
into 360 equal parts, called degrees ; if, therefore, 
we can find the length of one of these divisions, 
we can, -by multiplying it by 360, compute the 
length of the whole circumference. 

Now it is found, if we travel directly north, 
that the pole star approaches the zenith, and this 
approach can be measured by an instrument. 
Let the place then be accurately ascertained at 
which the pole star is one degree nearer to the 
zenith than at the place where we first commenced 
our observation. Now let the distance between 



MAGNITUDE OF THE EARTH. 27 

the places be measured, and we have the length of 
a degree on the earth's surface ; this is found to 
be 69| English miles, nearly ; which, multiplied 
by 360, gives a little more than 25,000 miles. 

It has been mentioned that the earth is not pre- 
cisely globular. Its form is called an oblate 
spheroid. Its equatorial diameter is 7925 miles ; 
the polar diameter is 7898, and its mean or ave- 
rage diameter 7912 miles. The polar diameter 
is, therefore, to the equatorial in the ratio of 301 
to 302, and the difference between them in length 
about 27 miles. In this statement fractions are 
not taken into account. 

QUESTIONS. 

What form has the surface of the sea in a calm ? What 
are some of the proofs of the earth's convexity ? What do 
we find to be the shape of the earth ? 

By knowing the length of a degree of a circle, how may 
we compute its circumference ? How do we find the length 
of a degree on the earth's surface? How long does it 
prove to be ? What then must be the circumference of 
the earth ? What is its form called ? What is its equa- 
.orial diameter ? What its polar ? What is the difference 
between them ? 



SECTION IV. 

MOTION OF THE EARTH ROUND ITS AXIS 
AND IN ITS ORBIT. 

The sun, moon, and stars, to our senses, rise 
in the east and set in the west. 

This appearance is occasioned either by a real 
motion of these bodies from east to west, or by 
the rotation of the earth in a contrary direction, 
that is, from west to east ; and this latter supposi- 
tion is the most probable. 

First. On account of the vast distance of the 
heavenly bodies from the earth, and consequently, 
the great swiftness with which each of them must 
move to perform a daily revolution round the earth ; 
for if the earth were at rest, the sun, which is one 
of the nearest, would have to move through twelve 
millions of miles an hour ; and vast as this rapi- 
dity is, it is absolutely nothing compared with that 
velocity with which the nearest fixed star would 
have to be whirled through space : while the same 
appearances might be presented if we supposed 
the earth to revolve round its axis in twenty-four 
hours, giving to the equator a motion of about 
1050 miles an hour, while the other parts of the 
earth would move with a velocity still smaller. 

(28) 



MOTION OF THE EARTH. 29 

Secondly. On account of the form of the earth, 
which is precisely of that shape it ought to have, 
did it revolve. 

Thirdly. On account of the diminution of the 
effect of gravity, or the attraction of the earth, as 
we approach the equator. 

For if the earth were at rest, since there is more 
matter at the equator than at any other part, the 
attraction or gravity ought to be greater there 
than at any other place. 

But it is found, on the contrary, that the pendu- 
lum oscillates more slowly, and that bodies weigh 
less at the equator, than in places near the poles ; 
and this effect is satisfactorily accounted for by 
the rotation of the earth round its axis ; for the 
parts of a body so revolving always tend to re- 
cede from the axis, and this tendency is greatest 
when the distance is greatest. Now, the parts 
about the equator are at the greatest distance from 
the earth's axis, and therefore the tendency to re- 
cede from the axis, and to diminish gravity or 
attraction, ought to be the greatest here, and so it 
is found to be. Also, all computations founded 
upon the existence of this tendency to recede from 
the axis, called centrifugal force, are correct ; and 
as this force upon the particles of a body attends 
rotatory motion, and no other, we are led to the 
3* 



30 OUTLINES OF ASTRONOMY. 

conclusion that the earth does revolve round an 
axis passing through the poles. 

But, beside the motion round its axis, the earth 
has another motion round the sun, which it com- 
pletes in 365 days, 6 hours, nearly. That the 
motion is that of the earth round the sun appears 
probable, because, in the first place, the sun is by 
far the larger body ; for, large as the earth appears 
to us, it is less than a millionth of the magnitude 
of the sun. 

The sun, indeed, appears to move among the 
fixed stars ; but it makes no difference in the ex- 
planation of phenomena, whether we suppose the 
earth to move round the sun, or the sun to move 
round the earth, and, therefore, reasonings built 
upon the apparent motion of the sun are groundless. 




For, if a be the place of the earth, a spectator 
looking at the sun at s, will refer it to some place 
in the heavens a among the fixed stars ; and when 



MOTION OF THE EARTH. 31 

he has arrived at b, perfectly unconscious of his 
own motion, he will look at the sun, s, at rest, and 
refer it to a point b, just as if the sun had really 
set out from a, and moved to b, and he had re- 
mained at rest. 

Another reason for the motion of the earth round 
the sun is, that the planets appear sometimes sta- 
tionary, sometimes moving in one direction, and 
at other times in a contrary direction, which ap- 
pearances can be easily explained on the supposi- 
tion of the earth's motion round the sun. 







Thus, suppose s to be the sun ; e, f, g, three 
places of the earth in its path ; a, b, c, three 
places of a planet in its path round the sun, at 
the same times. 

Draw the lines e a a, f b b, g c c. 

Then when the earth is at e, the planet appears 
at a, and when the earth has moved to f and the 



32 



OUTLINES OF ASTRONOMY. 



planet to b, the latter will appear at 6, or as if it 
had moved in the same direction through a b; but 
when the earth is at g and the planet at c, it will 
appear at c, behind a, and therefore will seem to 
have moved backward through b and a. 

Now these appearances are constantly occurring, 
and they cannot have an explanation without sup- 
posing the earth's motion. 




For, if the earth were at rest, and the planet 
moved round it, then if the planet be at a, it would 
appear to be at a, when at b, at &, and when at c, 
at c, or to move always in the same direction a b c. 

The points ab c, are among the fixed stars, 
among which, when we look at the fceavens, we 
suppose all the planetary bodies to lie. 

The earth then moves round the sun ; and its 
orbit is an oval, or ellipse, having the sun in a par- 
ticular point called the focus. 



MOTION OF THE EARTH. 33 




In this figure, e is the earth, a e p its orbit of 
an elliptic or oval shape, s the sun. 

a and p are the places at which the earth is at 
the greatest and least distances from the sun : on 
this account, a is called the aphelion, or from the 
sun ; and p the perihelion, or near the sun. 

QUESTIONS. 

Why is it more probable that the earth revolves on its 
axis, than that the sun and stars revolve about the earth ? 
What would be the velocity of the sun if it revolved daily 
round the earth? What is the rate of the earth's motion 
at the equator ? What is a second reason for supposing 
that the earth revolves on its axis ? What a third ? If the 
earth did not revolve, where should gravity be greatest ? 
But where is it greatest ? 

What other motion has the earth beside the one on its 
axis ? In what time does it complete this ? Why does it 
appear more probable that the earth moves round the sun, 
than that the sun moves round the earth ? 

What is the shape of the earth's orbit? What is the 
point within it called in which the sun is situated ? What 
is meant by the term aphelion ? By the term perihelion ? 






SECTION V. 
THE SUN. 

The sun, the source of light and heat to the 
earth, and the other planets which revolve round 
him, is a vast sphere, whose diameter is about 
888,000 miles ; its size is therefore nearly one 
million four hundred thousand times greater than 
that of the earth. 

The moon is 240,000 miles from the earth, a 
distance which is to us almost inconceivable. Yet 
if the centre of the sun were placed in the situa- 
tion which the centre of the earth now occupies, 
the surface of that enormous body would extend 
more than 200,000 miles beyond the orbit of the 
moon. Again, a man travelling at the rate of 
one hundred miles a day, would be able to go 
round the earth in rather less than three quarters 
of a year ; but it would require seventy-five years 
to travel round the sun's equator with the same 
speed. 

It is impossible to decido what is the composi- 
tion and nature of the body of the sun, but it 
seems to be surrounded by an ocean of flame. 

Black spots of an irregular form may be ob- 

(34) 



THE SUN. 35 




View of the Spots in the Sun. 

served on the surface of the sun, which are found 
to cross it from east to west in fourteen days. 

Their number, position, and magnitude are ex- 
tremely variable. Some have been observed whose 
diameter was more than fifty thousand miles. 

Sometimes, however, though but rarely, the sun 
has appeared free from spots for several years to- 
gether. When a spot is first discovered on the 
eastern limb of the sun, it appears like a fine line ; 
its breadth augments as it approaches the middle 
of the disk, and it diminishes as it goes over to 
the western limb, where it disappears. 

The same spot, after the lapse of thirteen days 
and a half, reappears on the western limb. This, 
however, is not always the case, as the spots dis- 



36 OUTLINES OF ASTRONOMY. 

solve and perish from causes unknown to us. 
Many of them undergo a daily change, and dis- 
appear in a short period ; sometimes a number of 
small spots unite into a larger one, and at other 
times a larger separates into a number of smaller 
ones, which soon entirely disappear. 

The spots are almost always surrounded by a 
penumbra, or shaded rim, which is enclosed in a 
cloud of light more brilliant than the rest of the 
sun, and in the midst of which they are seen to 
form and disappear. " This," says Laplace, the 
great French astronomer, " indicates that at the 
surface of this enormous mass of fire, vivid effer- 
vescences take place, of which our volcanoes form 
but a feeble representation." These spots have 
been serviceable in determining the rotation of the 
sun, and it is found that he makes a complete re- 
volution round an axis in twenty-five days and ten 
hours. 

The speculations concerning the nature of the 
sun are various. The opinion of Laplace, that it 
is a mass of fire, is not generally received. Many 
suppose that the sun is a dark body, surrounded 
by an enormous atmosphere, filled with luminous 
clouds, occasionally opening and exhibiting the 
dark mass within, and that the spots are portions 
of the solid sphere of the sun. 



THE SUX. 37 

On account of the real motion of the earth, the 
sun has an apparent motion among the stars ; the 
path which it describes is called the ecliptic, and 
is inclined to the heavenly equator, that is, is not 
parallel with it. When the sun comes to the me- 
ridian of an observer, it is the time of his mid- 
day, but the sun does not always cut the meridian 
at the same point, for if his altitude be taken at 
noon, it will be found to vary from day to day. 

After the 20th of March, when the sun is in the 
equator, he will be found to reach higher altitudes 
each day till about the 21st of June, when his 
altitude will vary but slightly for a few days ; on 
this account, this time is called the Summer sol- 
stice, or the standing still of the sun. From this 
time to the 21st of December, the altitudes of the 
sun will decrease, until he has reached his lowest 
meridian altitude, when he will again be nearly 
stationary for a few days ; this is called the Win- 
tor solstice. 

After this he will continue to ascend on the me- 
ridian till he again reaches his highest point. 

At or near the 20th of March and 23d of Sep- 
tember, the sun being half-way between his greatest 
and least altitudes, he is in the equator ; and, as 
will be shown, the days and nights are then equal 
over the whole terrestrial globe. 
4 



38 OUTLINES OF ASTRONOMY. 

QUESTIONS. 

What is the diameter of the sun ? How much larger 
than the earth is it ? How long would be required to 
travel round the earth at the rate of one hundred miles a 
day ? How long to travel at the same rate round the sun ? 
I What are seen at times on the surface of the sun ? In 
what direction do they cross it ? In what time ? How 
large have some of them been seen? How long has the 
sun sometimes appeared free from spots ? How do they 
appear and disappear ? What changes do they undergo ? 
In what have these spots been serviceable ? In what 
time does the sun complete a revolution on its axis ? 

What was the idea of Laplace respecting the sun ? What 
do others suppose ? 

What is the path which the sun describes among the 
stars called? What do you mean when you say that it is 
inclined to the heavenly equator ? 

Where is the sun on the 20th of March ? When is the 
summer solstice ? Why is it so called ? When does the 
sun reach its lowest meridian altitude ? What is its place 
then called ? When the sun is on the equator, what is the 
relative length of the days and nights ? 



SECTION VI. 
DAY AND NIGHT.— THE SEASONS. 

We proceed to explain the causes of day and 
night, and of the seasons; of that wonderful 
change from light to darkness, from summer to 
winter, so conducive to the happiness and exis- 
tence of man. 

In the ordering of these events, we trace the 
hands of an Almighty Creator, ever watchful over 
the wants and comforts of the human race. 

The rotation of the earth round its axis, by 
which each portion of the surface of the globe is 
successively presented to and removed from the 
sun, is the cause of day and night. 

The inclination of the earth's axis to the orbit 
which it describes, and its constant direction to the 
same point in the heavens, viz., the pole, afford 
us the agreeable changes of summer and winter, 
spring and autumn. 

The orbit of the earth, or the path it describes 
round the sun, is very nearly a circle, and the sun 
is nearly in its centre. 

If the axis of the earth, or the line round which 
it turns, were perpendicular to its orbit, or, what 

(39) 



40 OUTLINES OF ASTRONOMY. 

is the same thing, to the plane in which the sun 
is, the parts about the equator would be constantly 
exposed to the full effects of the sun's rays, and 
would be burnt up with intolerable heat. 

The day and the night would be equal over the 
whole globe — the same season would perpetually 
reign. 

To the inhabitants of our latitude, a constant 
and cold spring would be exchanged for the happy 
vicissitudes of the seasons ; while, as we approach 
the poles, a perpetual winter would destroy all 
vegetation, although, for twelve hours, the rays of 
the distant sun would shine upon the scene. 

At the poles the sun would neither rise nor set, 
but would perpetually revolve in the horizon. 

This effect may be easily exhibited by means 
of a common globe. 




s 
Place a lamp so that its wick may be of the same 
neight as the equator, at a considerable distance 



THE SEASONS. 



41 



from the globe ; then, as the globe is turned round, 
the illuminated parts will be those which are about 
the equator, while the poles and a considerable 
part of the globe in their neighbourhood will be in 
the shade, which shaded part will become less as 
we remove the lamp from the globe. 

In order to explain the changes of the seasons, 
we must have recourse to a figure, and at the same 




time we shall be enabled to show the difference of 
the lengths of days in the different periods of the 
year. 

Let o be the sun, w v s a the orbit or path of 
the earth, v and a being the vernal and autumnal 
equinoxes ; s and w the summer and winter sol- 
stices ; n s the axis of the earth ; n being the 
north and s the south pole, and s n always pa- 
rallel to itself: suppose the circular figures drawn 

at w s, v n, to be upright on the paper like shillings 

4* 



42 



OUTLINES OF ASTRONOMY, 



on their edges, and to be placed flatways to an 
eye looking from the bottom of the paper to the 
top ; then the edges of these circles at w and s 
will pass through the sun, and their flat sides at v 
and a will be turned towards him. 

At w, which is our winter, we see that n, the 
north pole, is turned from the sun, while at s, the 
place of our summer, n is turned towards him. 

Carrying, therefore, the figure in the memory, 
we must, to explain it fully, take each case sepa- 
rately. 

To begin with the summer solstice. 







%,T^ 



Suppose the above figure to represent the earth, 
N its north pole, s the south pole, e q, the equator, 
and o the sun. 

Now, since one-half the globe is always illumi- 
nated by the rays of the sun, let l d separate the 
dark part from the region of light ; then, in this 
position of the earth, the north pole has constant 
day and no night, while the south pole, and the 



DAY AND NIGHT. 43 

country immediately about it, receive none of the 
sun's rays. 

For, if we suppose the figure to turn round n s 
while n s remains fixed, any points between l and 
n will always be on the right hand of l d, or con- 
stantly within the portion of the earth illuminated 
by the sun ; while places between d and s will 
always be on the left hand of l d, and will con- 
sequently never emerge from the shade. 

Now, since the earth revolves round its axis in 
about 24 hours (23 hours 56 minutes), a point a, 
will be carried to a in twelve hours, and again to a 
after another twelve hours, having passed through 
its night and day ; a being that part at the greatest 
distance from the sun, is the place of midnight, c 
the place at which it emerges into the light, the 
sun-rise; and a, the place opposite the sun, its 
noon or mid-day. 

In the same manner, if b be another place on 
the same meridian, b, c, &, will be its midnight, 
sunrise, and noon respectively. 

Now we all know that from midnight to morn- 
ing is half the night, and from morning to noon is 
half the day. Hence a c will represent half the 
night and c a half the day. But here a c is ma- 
nifestly less than c a, or the night is less than the 
day; or in the northern half of the world, when 



44 



OUTLINES OF ASTRONOMY. 



the sun is above the equator, as in this figure, the 
day is longer than the night. 

Let us now turn to b, which is below the equa- 
tor, then b c will represent half the night, and c b 
half the day, but b c is plainly greater than c b ; 
or the night in southern latitudes, in this position 
of the earth, is longer than the day. 

Secondly, if we conceive the earth to be in the 
opposite part of its orbit, and therefore at the win- 
ter solstice, its appearance is represented in the 
following figure. 




0|sl- 



Here the north pole is in the shaded part of the 
figure, and s the south pole, in that part always 
exposed to the sun ; and a line drawn from o to 
e a falls below e q, or the sun is below the equa- 
tor ; if now we take two places a and b, the one 
north and the other south of the equator, it is im- 
mediately seen that the night to the persons situ- 
ated at a, is longer than the day, while to persons 



THE SEASONS. 



45 



at b the contrary takes place. To the inhabitants 
of the regions at and near the north pole, the night 
is ever present, while to the south pole the sun is 
constantly visible. 

At the vernal equinox the boundary of light and 
darkness will pass through both poles, and the re- 
presentation will be as follows. 






The sun being supposed above the figure, a a and 
b b will represent the day at each of the places a 
and b, and since these lines are twelve hours, the 
night is as long as the day, and this is true at all 
parts of the earth. Hence the origin of the term 
equinox. 

At the autumnal equinox, the circle of light and 
darkness also passes through the poles, and a simi- 
lar figure may be used. 

It may be readily perceived, by reference to the 
figures, that at e or q, the day and night are always 
equal ; hence the name of equinoctial has been 
given to this circle. Hence it appears that the re- 



46 OUTLINES OF ASTRONOMY. 

volution of the earth in its orbit, and the perma- 
nent inclination of its axis, produce the changes 
of the seasons, and that the daily revolution round 
its axis fully explains the cause of day and night. 

That the rigor of our winter is chiefly owing 
to the northern parts of the earth being then at 
their greatest inclination from the sun, and the 
heats of summer to their being turned towards 
him, and that it is this inclination from or towards 
the sun, and the consequently different altitude 
which the sun reaches above the horizon, which 
causes the difference of temperature, is proved by 
the fact that the sun is absolutely nearer to us in 
winter than in summer. 

It may be remarked that, at the equator, the 
poles appear in the horizon, and that at the north 
pole, the pole star is in the zenith, and the rational 
horizon coincides with the equator. 

For the purpose of illustration, circles, instead 
of spheres, have been used in the figure explana- 
tory of the four positions of the earth ; for, by the 
circles, the angle made by a line drawn from the 
sun to the centre of the earth's axis, on which the 
alteration of the seasons so greatly depends, is 
more easily conceived. This line is perpendicular 
to the earth's axis at the equinoxes, and inclined 
to it at the solstices. The whole figure might be 



THE SEASONS. 




easily presented to the eye, by means of four 
circular pieces of pasteboard, about an inch in 
diameter, and attached to a string arranged in a 
circular form, taking care that the line ns is 
always parallel to itself. 

The above figure will exhibit the four positions 
of the earth considered as a sphere. 

QUESTIONS. 

What is the cause of day and night ? What is the cause 
of the change of seasons ? What is the shape of the 
earth's orbit? If the earth's axis were perpendicular to 
its orbit, what would occur at the equator ? What in our 
latitude ? What at the poles ? 

In what part of the heavens do the poles appear to an 
observer at the equator ? Where does the pole star 
appear to an observer at the north pole ? With what 
would the rational horizon coincide to the same observer ? 



SECTION VII. 

ATMOSPHERIC REFRACTION. 

The earth is surrounded by an atmosphere of 
variable density ; the consequence of which is, that 
when a ray of light proceeding from a star, or 
other luminous body, enters the atmosphere, it no 
longer pursues its original direction, but is bent 
towards the earth. Thus, suppose s to be a star, 




s e a ray of light, and let the shaded part repre- 
sent the atmosphere, first supposed to be of uni- 
form density, and e the place at which the light 
enters it ; then, instead of going from e to s, it 
will be bent into some direction e a, and if a be 
the eye of a spectator, the light coming in the 
direction e a, will make him suppose that it came 
from a star at t : this effect is called refraction. 
Hence, by refraction, the stars are elevated 

(48) 



ATMOSPHERIC REFRACTION. 49 

above their true places, and it always affects them, 
except when they happen to be in the zenith. Its 
greatest effect is when the stars are in the horizon, 
and by it we are enabled to see the stars and the 
sun when they are actually below the horizon. 

In the figure to which we have just referred, the 
light, after entering the atmosphere, is supposed to 
move in a straight line, e a, directly to the eye. 
This, however, is not quite true, for the light does 
really describe a curve, on account of the refrac- 
tion of the atmosphere being different at different 
altitudes. For refraction wholly depends on the 
density of the atmosphere, and the density dimi- 
nishes as we ascend from the earth. In the figure, 
let the interior sphere represent the earth, the outer 
circle the limits of the atmosphere ; and let the 
circles between them separate the strata of the at- 
mosphere which differ in density from each other ; 
now, since the density decreases as we ascend, the 




50 OUTLINES OF ASTRONOMY. 

density al a is greater than that at b, and that at 
b greater than that at c. 

That a star apparently rises before it actually 
reaches the horizon, may be seen by attending to 
the figure. Let a be the place of a spectator, a t 
a plane touching the earth at a, and consequently 
his horizon ; let s be a star, and sea ray of 
light, entering the atmosphere at c, and proceeding 
from s. 

■ Then, by the outer rim of the atmosphere, it is 
bent into the direction cb ; by the next rim, which 
we call b, it is further bent into b a, and, by the 
last portion of the air, it is refracted into the direc- 
tion a a, and enters the eye at a, as if it came 
from a star at t ; thus, while the star is at s, it is 
thought to be at t. 

Another important property of the atmosphere 
is its power of reflecting light. This is the cause 
of twilight, which prevents us from being plunged 
into instant darkness upon the first absence of the 
sun ; for when the sun is half a degree below the 
horizon, the refraction is not great enough for its 
rays to come to the eye, but they pass over our 
heads, and are reflected irregularly by the par- 
ticles of the atmosphere. And although we do not 
see the sun, yet a part of the sky is illuminated, 
and reflects its borrowed light to us. 



ATMOSPHERIC REFRACTION. 



51 



Repeated observations prove to us that we have 
some light from the sun for an hour and twelve 
minutes after he has set. But this duration of the 
twilight varies in different latitudes ; at and near 
the equator the twilight is always very short ; at 
the poles it lasts for several months ; at the north 
pole from September 22d to November 12th, and 
from January 25th to March 20th. At some times 
of the year the twilight lasts all night ; this hap- 
pens in England in the month of June. 

Another effect of refraction is the oval figure 
of the sun and moon as they approach the horizon. 

The upper half being less raised than the lower 
by nearly one-sixth of the whole, while the diame- 
ter parallel to the horizon remains the same, the 
bodies assume an oval form. 






Thus, if ad cb be the face of the moon unaf- 
fected by refraction ; 



52 OUTLINES OF ASTRON03IY. 

dc its vertical diameter ; 

ab the horizontal diameter; 

Then the refraction will merely elevate ab to 
ab, but the point c will be raised to c, and d to d, 
where c c is greater than d d, and the moon will 
be of the shape adbc, which is an oval and not a 
circle. 

The great apparent magnitudes of the sun and 
moon when in the horizon must strike every eye. 
The probable cause of this, is not that they make 
a greater angle at the eye, but the wrong judg- 
ment we form of their distances then, compared 
with their distances when seen higher. In conse- 
quence of the variety of intervening objects, we are 
led to judge their distance to be greater than when 
elevated ; and as their apparent diameters are 
about the same in both cases, we are apt to ima- 
gine that object to be the largest, the distance of 
which we conceive greatest. Of this we may 
easily satisfy ourselves by looking at the sun when 
it is near the horizon, through a smoked glass ; as 
we shall then see no other objects, no such differ- 
ence will be observed in the size of the sun. In- 
deed if any difference could be observed, the sun 
and moon should appear smaller near the horizon 
than at the zenith, for they are then more distant 
by half the diameter of our globe. 



ATMOSPHERIC REFRACTION. 53 

We can easily ascertain the weight of a column 
of air of any given thickness, as for instance of a 
square inch, and of the whole height of the atmo- 
phere, by weighing the column of mercury which 
a column of air will balance, as it does that of the 
barometer. With the aid of this knowledge ft is 
determined, that if the upper portions of the atmo- 
phere were as dense as that portion which is 
nearest to the earth, the height of the whole would 
not be much over five miles; that is, the atmo- 
sphere would be about equal in height with the 
highest mountains. 

But the upper portions, being less compressed, 
are necessarily rarer ; and it is proved by the 
height at which light is reflected, that the atmo- 
sphere must extend upward at least forty-five miles. 
As the power of the atmosphere to reflect light 
decreases in proportion to the decrease of its den- 
sity, the appearance of the sky as seen from a 
very high mountain, is different from that pre- 
sented at the ordinary surface of the earth. It 
there looks dark, or even black ; and the stars are 
visible in the day time. 

By examining a rainbow or looking through a 
prism, we can see that rays of light may be sepa- 
rated, and arranged according to their different 
colors. This separation and arrangement depend 
5* 



54 OUTLINES OF ASTRONOMY. 

on the fact that the rays of some colors are more 
refracted or bent than others, in passing from one 
medium to another that is differently constituted. 
Some of the colors which we see are primitive ; 
the others are mixtures of these in various propor- 
tions. Newton supposed the number of primitive 
colors to be seven, and his theory has been gen- 
erally adopted. When arranged according to their 
refrangibility, or the degree to which they are re- 
fracted, they occur in the following order, the most 
refrangible being given first : 

Red, 

Orange, 

Yellow, 

Green, 

Blue, 

Indigo, 

Violet. 
Sir David Brewster, however, and some others, 
suppose that there are but these three primitive 
colors ; Red, Yellow, and Blue ; and that all the 
others are various compounds of these. The ordi- 
nary light of the sun, containing all the colors in 
a certain ratio, is called white light. 

In some conditions the atmosphere is most fa 
vorable to the transmission of one of these colors, 
and in some to that of another. Hence the sky is 



ATMOSPHERIC REFRACTION. 55 

of different colors at different limes ; as, more or 
less red at sunrise and sunset, and more or less 
blue at other times. 

QUESTIONS. 

By what is the earth surrounded ? What is the effect of 
the variable density of the atmosphere on a ray of light ? 
What is this bending of the direction of the rays of light 
called ? What effect has refraction on the stars ? When are 
they not affected ? When is the effect of refraction greatest ? 
What does it then enable us to do ? 

What is another important property of the atmosphere ? On 
what does twilight depend ? How long does twilight last 
in our latitude ? Is it the same in all latitudes ? What is 
its duration at the poles ? 

What is another effect of refraction ? 

What is the cause of the great apparent magnitudes of the 
sun and moon when they are near the horizon ? Are they 
any larger when on the horizon than when in the zenith ? 
How may we convince ourselves of this ? If we could per- 
ceive a difference in them in different situations, where 
should they appear smaller ? How much more distant are 
they on the horizon than in the zenith ? 

How may we determine the weight of a column of the at- 
mosphere ? How high would the atmosphere be if it were 
of equal density throughout ? How high does the reflection 
of light prove it to be ? What is the appearance of the sky 
from a very high mountain ? 

What is proved by the rainbow and the prism ? On what 
do this separation and arrangement depend? What two 
classes of colors are there ? How many primitive colors 
are there, according to Newton's theory ? According to 
what are they arranged, and in what order do they occur ? 
What is the opinion of Sir David Brewster and some 
others on this subject? What is the ordinary light of the 
sun called ? Why is the sky of different colors at different 
times ? 



SECTION vni; 
THE MOON. 

Next to the sun, the most interesting object to 
us, among the heavenly bodies, is the moon. The 
dazzling splendor of the sun renders a full view 
of it insupportable to human vision ; but the 
brightness of the moon presents a pleasing object 
to the eye, and its silvery light relieves with a 
cheering influence the dulness of our long winter 
nights. Its changes from the thin crescent to the 
full orb ; its waning from its full to its crescent 
again ; must attract and have attracted the atten- 
tion of thoughtful minds in all ages. The un- 
taught, always prone to mistake his works for the 
Creator himself, have, in almost every clime, 
offered to the moon their worship. The poet has 
addressed to her his hymn and song; and early 
science, unable to discover other causes, assigned 
to the moon an influence over the minds of men. 
Most of the ancient nations, especially the He- 
brews, Greeks, and Romans, assembled at the 
time of new or full moon, to express their pious 
gratitude for her constant attendance on the earth, 
and the many benefits she conferred on it. 

(56) 



THE MOON. <)i 

To the astronomer, the moon is an object of 
great importance. She affords the readiest means 
of determining the form of the earth ; she enables 
us to ascertain with some accuracy the distance 
of the sun, and thus furnishes a measure for as- 
certaining that of the planets, and those remote 
worlds, the fixed stars. By her the mariner can 
direct his course on the lonely ocean ; for, by cal- 
culating her motions, he can pursue his way with 
as much certainty as the traveller feels when fol- 
lowing his path by her light on land : and she is 
the chief cause of the tides — those wonderful move- 
ments of the waters of the earth, so essential to 
their purity, and so important in the intercourse 
of man with man. 

When the moon is viewed through a telescope, 
several curious phenomena present themselves. 
Spots are observed differing very greatly in their 
degrees of brightness. Some are almost dark. 
Many of them seem to be vast excavations in the 
moon's surface. Some of these must be three or 
four miles in depth, and thirty or forty miles in 
diameter. 

Water does not seem to exist on the surface of 
the moon, at least not in large quantities, since no 
changes have ever been observed upon her surface, 
such as would be produced by the presence of va- 



58 OUTLINES OF ASTRONOMY. 




View of the Moon. 

pors or clouds ; for although the atmosphere of 
the moon, if there be one, is of very small extent, 
yet it is very probable that one does exist, and if 
there were vapors raised by the sun's heat, they 
would be seen floating in it, and obscuring the face 
of the moon. 

That there are mountains in the moon, is made 
very evident by a number of bright spots which 
are seen in the dark part of the face, near to the 
separation of light and darkness. 

These are the tops of eminences, enlightened 



THE 310 OX. 



59 



by the rays of the sun, while their lower parts are 
in darkness. 

The whole surface of the moon, on the side 
which is visible to us, presents a most wild and 
rugged aspect, which is scarcely equalled by the 
grandest mountain scenery of our globe. There 
are four principal classes of mountains in the 
moon : 

First. The Insulated Mountains, which rise from 
the plains like a sugar-loaf placed on a flat sur- 
face, and which remind one of Mount Etna, and 
the Peak of Teneriffe. 

Second. Ranges of mountains, some of which 
are several hundred miles in length, and bear a 
striking resemblance to the Alps, Andes, and other 
ranges ; a circumstance which has determined the 
choice of the names which have been applied to 
them. 

Third. Circular Ranges, to which there is no- 
thing similar on this globe. They are of great 
extent and height, sometimes rising from the plain, 
and at others forming a sort of huge rim to the 
excavations which we have mentioned. 

Fourth. Central Mountains. These rise from 
the circular plains and excavations, in a variety 
of shapes, sometimes to the height of a mile and 
a half, and frequently throw a very distinct sha- 



60 OUTLINES OF ASTRONOMY. 

dow, by which we are enabled to calculate their 
elevation. 

Some idea may be given of the method by which 
the height of mountains in the moon is known, 
from the following process : — 

Suppose that the height of the mountains, as- 
certained by aid of a micrometer (an astronomical 
instrument for measuring small angular distances), 
be 3 seconds ; by the same instrument the dia- 
meter of the moon is 32 minutes, or 32 X 60 
seconds, or 1920 seconds ; therefore, these mount- 
ains are but 19 3 20 or ^-^th part of the diameter 
of the moon. 

If the diameter of the moon is 2000 miles, the 
height of the mountain is 6 ^o tn P art °f 2000 miles, 
or a little less than three miles and a quarter. 

But sometimes bright spots are observed in the 
darkened half of the moon, which could not be 
illuminated by the sun in consequence of their 
distance from the light part. By some astronomers 
these are supposed to be volcanoes in an active 
state of conflagration ; the height of one of these 
was measured by Dr. Herschel, and found to be 
four miles. 

The moon always presents the same side to the 
earth. Hence, she must revolve on her axis in pre- 
cisely the same time in which she revolves round 



THE MOON. 61 

our globe, or in about twenty-seven days and a 
half. A day, therefore, in the moon, is equal to 
nearly two weeks with us ; and a day and night 
little shorter than one of our months. 

Although the moon seems second only to the sun 
in magnitude and brightness, she is the smallest of 
the heavenly bodies that can be seen with the 
naked eye. Her diameter is two thousand one 
hundred and sixty miles, or a little more than one 
fourth that of the earth, which therefore exceeds 
her forty-nine times in bulk. 

The distance of the moon from the earth is two 
hundred and forty thousand miles, or about thirty 
times the earth's diameter. This is not more than 
a four-hundredth part of the sun's distance. This 
explains why the moon appears so large, though 
it would require seventy millions of such bodies 
to form a globe as large as the sun. 

The light which the moon sheds on the earth 
does not originate in that body, but is reflected 
from the sun. It has been calculated that to give 
us as much light as we receive from the sun, three 
hundred thousand such moons as ours would be 
required. 

As the moon sheds light upon the earth, so does 
the earth at times enlighten the moon ; and if 
there are inhabitants on that body, as we have 
6 



62 OUTLINES OF ASTRONOMY. 

some reasons for believing, the earth, passing 
through its various phases in their sky, and once 
in each month, when it is full, shining with a sur- 
face thirteen times as large as the full moon, must 
present to them a magnificent spectacle. 




The Earth as seen at the Moon. 

As the inequalities in the moon's surface cause 
it to appear spotted to us, so the geographical 
divisions of the earth are visible to the lunar na- 
tions. When the earth is full to them, they may 
see its continents and oceans pass successively 
over its surface, at the rate of fourteen times in 
one of their days. 



THE MOON. (33 

As the atmosphere of the moon does not seem 
adapted to the formation of clouds, these appear- 
ances, and the changes which the huge face of the 
earth undergoes in its increase and decrease, may 
be observed for hundreds of hours at a time, and 
must be more calculated to excite inquiry, than 
any astronomical scene that is presented to our 
eyes, while unassisted by the telescope. 

The moon moves round the earth at the rate of 
two thousand two hundred and ninety miles in an 
hour ; her orbit is of a very singular figure, and 
the calculation of her motions is a very compli- 
cated process, owing to the number of circum- 
stances which must be taken into consideration. 
The principal of these are the forces with which 
the earth and the sun attract her. Notwithstand- 
ing the great distance of the sun, the attractive 
power which it exerts over the moon is two and 
one-fifth times greater than that of the earth. 
The moon, therefore, moves in such a manner as 
always to present a concave side of its orbit to the 
sun, while revolving round the earth, and proceed- 
ing with the earth round the sun. 

The reason why the moon appears to us to rise 
and set is, that in the morning one side of the 
earth is turned to her, and in the evening the 
other. It has been mentioned that the moon makes 



64 OUTLINES OF ASTRONOMY. 

a revolution on her axis in the time in which she 
revolves round the earth. A little reflection on 
these facts will convince the learner, that to the in- 
habitants of the moon the earth must seem to re- 
main stationary in the sky, though the sun and all 
the stars appear to make monthly revolutions. 

From observations which may easily be made 
by any person, it is found that the moon has a 
motion from west to east ; this may be proved by 
looking at the moon upon any clear evening, and 
observing its distance from some brilliant star ; 
the next evening, the distance between the moon 
and star will be considerably altered : if the star 
be eastward of the moon, the apparent distance 
between them will be first diminished, then the 
moon will pass by the star, and afterwards the 
distance will become greater ; but if the star be to 
the west of the moon, the distance will for many 
evenings continue to increase. After twenty-seven 
days seven hours and forty-three minutes, the 
moon and star will be at precisely the same dis- 
tance as at first : this time is not, however, what 
we call a month, which is the interval between 
new moon and new moon, and is twenty-nine 
days twelve hours and forty-two minutes, nearly. 
This difference is caused by the motion of the 
earth in her orbit, as will be seen from the annexed 
figure. 



THE MOON. 65 




Let s be the sun, e the earth in her orbit, n o 
the orbit of the moon round the earth. 

Now new moon takes place when the moon is 
between the earth and the sun, for then her dark 
part is entirely turned towards us, and full moon 
is when the earth is between the moon and the 
sun, as then we see the whole enlightened part ; 
but it may be asked, why does not the earth ob- 
struct the light of the sun, and prevent it from 
falling on the disk of the moon, and so we may 
never have full moon? To this we answer, that 
sometimes this does happen, and then we have an 
eclipse ; and it would always happen, but that the 
path of the moon is sometimes elevated above the 
earth's orbit, and sometimes depressed below it, so 
that the sun shines upon the lunar disk. 

Now let n and o be the place of the new and 
full moon ; then when the moon is at full, a spec- 
6* 



66 OUTLINES OF ASTRONOMY. 

tator at e, looking at it, would see a star s exactly 
in the same direction ; possibly the moon might 
pass over and hide the star; then if the earth 
were at rest after twenty-seven days and seven 
hours, he would see the moon and star in the same 
position : but the earth moves ; let then the earth 
be at f, at the time when the moon and star are in 
the same direction, join f s, it will cut the moon's 
orbit in q ; but p is the place of full moon. To 
get to p, the moon must travel over q p, and a 
little more, before the month is completed, that is, 
before another full moon ; so that the difference 
between the time of the moon's revolving round 
the earth and the lunar month is caused by the 
motion of the earth ; and hence also may we con- 
clude that the moon moves round the earth, and it 
is found that its orbit is very nearly a circle, having 
the earth almost in its centre. 

We proceed to explain some peculiarities of the 
rising and setting of the moon at various seasons. 

The rising and setting of the moon is most in- 
teresting at or near the full moon. At full moon, 
as we have seen, it is opposite the sun, and in or 
nearly in the same plane; when the sun is above 
the horizon, the moon is below, and when the 
moon is visible the sun is no longer so. 

Hence at midsummer, when its light is least 



THE MOON. 67 

wanted, the moon at full is very little above the 
horizon. In midwinter at full, when the sun is 
visible but a short time, the moon being in the 
opposite side of the plane, remains long above the 
horizon, and the quantity of moonlight is greatest 
at a season when its presence is most wanted ; and 
this is more remarkable as we approach the poles ; 
at that region, in midwinter, the moon does not 
set for fifteen days together. 

The moon rises later every day, this is called 
her retardation ; but this varies at different seasons 
of the year ; sometimes the difference is nearly 
an hour, sometimes only a quarter of an hour, 
and in the autumn the alteration of the time of 
rising is least. 

Hence we shall be able to give the explanation 
of that delightful phenomenon, the harvest-moon. 

At the autumnal equinox, as we have said be- 
fore, the moon is in that part of her orbit where 
the time of her rising on successive evenings alters 
the least ; in fact, its daily variation is not more 
than a few minutes for several days. Now the 
full moon being opposite the sun, the moon will 
rise as the sun sets, that is, about six o'clock in 
the evening, which is the time of sunset at that 
season of the year. The next evening it will rise 
only a few minutes after six, and so on a little 



68 OUTLINES OF ASTRONOMY. 

later for several successive evenings. This un- 
usual circumstance happening at a time when the 
industry of the husbandman is stimulated by the 
hope of gathering the rich reward of the harvest, 
the moon which is at or near the autumnal equi- 
nox is called the harvest-moon. In places near 
the poles, where the changes of season are more 
rapid than with us, this appearance is more re- 
markable, and of greater use to the farmer. It is 
not wonderful, therefore, that those engaged in ag- 
ricultural pursuits, noticed this adjustment earlier 
than astronomers, and ascribed it to the kindness 
of Providence. 

QUESTIONS. 

What object next to the sun is of most interest to us 
among the heavenly bodies ? What has attracted particular 
attention to her ? What are the several uses of the moon 
which make her important to the astronomer ? 

What are observed when we examine the moon ? What 
do many of the spots appear to be ? What must be the 
extent of some of them ? 

Why is it supposed that water does not exist on the sur- 
face of the moon ? 

By what is it made evident that there are mountains in 
the moon ? What is her general appearance ? How many 
classes of mountains are seen on her disk ? How are they 
distinguished? Describe the first class — the second — the 
third— the fourth ? 

What are other bright spots supposed to be ? How high 
did Dr. Herschel estimate one of these to be ? 



THE MOON. 69 

Does the moon present more than one side to the earth ? 
What then is the relative length of time in which each of 
her revolutions is made ? How long are the days and nights 
in the moon ? 

What is the diameter of the moon ? How much larger 
is the earth ? What is the moon's distance from the earth ? 
How much more distant is the sun? How much larger 
than the moon is the sun ? 

Where does the light of the moon originate ? How many- 
such moons as ours would be required to shed as much light 
on the earth as the sun ? 

What is the appearance of the earth when viewed from 
the moon ? When the earth is full to the inhabitants of 
the moon, how often do they see its oceans and continents 
pass over its surface in one of their nights ? Why may 
the earth's phases be viewed for a long time together by 
the lunarians ? 

At what rate does the moon move round the earth ? How 
much greater is the sun's attractive power on the moon 
than the earth's ? What effect has this on its orbit ? 
What is the effect of the moon's revolving once on its axis 
in the precise time in which it revolves round the earth ? 



SECTION IX. 
THE PHASES OF THE MOON.— ECLIPSES. 

Among the most striking phenomena of the 
heavens, may be reckoned the phases of the moon. 
By the phases are meant those changes of appear- 
ance in the lunar disk, which are presented to us 
in the course of one complete revolution of the 
moon. After passing through conjunction, when 
what we call new moon happens, it reappears with 
a slender crescent, which increases each evening, 
and becomes an entire circle of light when the 
moon is in opposition with the sun. 

After full moon, the circle is changed again into 
a crescent, which diminishes by the same degrees 
as it increased, till it entirely disappears. 

The crescent of the moon is always turned to 
the sun, and it evidently shows that it shines with 
light borrowed from the sun. 

The causes of these phases may be explained, 
although imperfectly, by a figure : a more com- 
plete representation may be made by means of a 
spherical ball, carried round the head of the ob- 
server, and illuminated by a lamp of nearly the 
same height, but in a distant part of the room. 

(70) 



PHASES OF THE MOON. 



71 




Let e be the earth, m, n, o, the orbit of the moon 
round it, and m, n, o, three positions of the moon, 
s n, and s h, rays proceeding from the sun, which 
are drawn in the same direction (i. e. parallel), 
because the sun is so very distant. 

Now one-half of the moon, viz. that which is 
turned towards the sun, is always bright, and the 
other half is dark. Let d l be drawn perpendi- 
cular to the lines s h, and s n, and it will divide 
the moon into two equal parts, one the bright, the 
other the dark part. 

But a spectator at e only sees one-half of the 



72 



OUTLINES OF ASTRONOMY. 



moon at the same time, and this half is found by 
drawing a line, g h, perpendicular to a line from 
e to the centre of the moon, but which, to avoid 
confusion, is not drawn in the figure. 

Now, at n, g h and l d are the same lines, and 
the part turned to e is the dark part, and no moon 
is there visible ; but when it gets to o, the bright 
part seen by e is l h, one-half of the enlightened 
part of the figure ; and since we must consider that 
the moon is a globe, and that only half of it is re- 
presented in the figure ; the whole appearance will 
be that of the moon at the end of the first quarter, 
or the moon is said to be dichotomized, or cut into 
two equal parts. At t, which is a place between 
the first and second quarters, the part visible to 
the earth is l h, which is considerably more than 
half the bright part. Both sides of the moon are 
convex, and it is then said to be gibbous ; the ap- 
pearance is represented in the figure a a b b, a a b 
being a semicircle, and a b b an ellipse. 




ECLIPSES. 73 

At m the line of vision g h and that of light and 
darkness d l are again coincident ; but the bright 
part is then visible, and we see the moon to shine 
with a full orb. After passing through m, the en- 
lightened part seen from e diminishes between if 
and p, but the illuminated part visible at the earth 
will be greater than half the disk, and it will ap- 
pear gibbous : at p we see only half the moon, and 
her third quarter is completed. 

After passing through p the thin crescent again 
appears, and gradually diminishes ; at r we see 
only a small part d g ; and at n the enlightened 
part is again entirely hid from the earth. 

The explanation of the phases of the moon na- 
turally leads to that of eclipses, formerly the ob- 
jects of terror, now merely of curiosity. 

An eclipse of the moon, or, as the word signifies, 
a deprivation of the light of the moon, can only 
take place by some dark body coming between it 
and the sun : this dark body is the earth ; for 
eclipses of the moon only take place when she is 
full, and then the earth lies between the sun and 
moon. The globe of the earth casts behind it re- 
latively to the sun a shadow of a conical shape, 
the length of which is three times the distance of i 
the moon from the earth; its breadth, where the 
moon crosses it, is about double the lunar dia-, 
meter ; and the direction of the middle of this cone 



74 OUTLINES OF ASTRONOMY. 

is a line joining the centres of the sun and earth. 
Now at the full, the moon is also in the same direc- 
tion ; she would therefore be eclipsed every month, 
if her orbit were not inclined to that of the earth ; 
but as it is inclined, the moon is sometimes above, 
sometimes below the shadow, and consequently 
this phenomenon does not always happen. If the 
whole disk is immersed in the shadow, the eclipse 
is said to be total ; and to be partial, when only a 
portion of the disk is obscured. 

An eclipse of the sun happens at new moon, for 
then the moon is between the earth and the sun, 
and the moon intercepts the rays of light proceed- 
ing from the sun, and hinders them from falling 
upon the earth. 

A material difference exists between lunar and 
solar eclipses — the former are seen at the same 
time, by every spectator who sees the moon above 
his horizon, the latter may be seen by one spec- 
tator and not by another. 

The shadow of a cloud transported by the winds, 
and rapidly passing over hills and valleys, de- 
priving those spectators it reaches of the light of 
the sun which others are enjoying, gives us an 
exact image of a total eclipse of the sun. 

Although the sun is so much larger than the 
moon, yet the latter is so much nearer us than the 



ECLIPSES. 



75 



former, that their apparent magnitudes differ but 
little ; they indeed surpass each other alternately. 

Let us suppose the sun and moon in the same 
straight line with the eye of a spectator, he will 
see the sun eclipsed, and if the apparent diameter 
of the moon be greater than that of the sun, the 
eclipse will be total ; but if less, a ring of light 
will be formed by that part of the sun which ex- 
tends beyond the disk of the moon, and the eclipse 
is said to be annular ; if the moon is not exactly 
in the straight line which joins the centre of the 
sun and the observer, the eclipse will be partial. 

The profound night which accompanies a total 
solar eclipse, lasts under the most favourable cir- 
cumstances about seven minutes and a half. The 
shortness of this period overthrows the conjecture, 
that the darkness, mentioned in the account of the 
Crucifixion, was caused by an eclipse of the sun. 

The number of eclipses that can happen in a 
year is seven, five of the sun and two of the moon, 
or four of the sun and three of the moon. 




76 



OUTLINES OF ASTRONOMY. 



In the foregoing figure a total eclipse of the 
moon is represented, s is the sun, e e the earth, 
m m the orbit of the moon, e e t is the conical 
shadow cast by the earth, and the black spot is the 
eclipsed moon. 

The moon's diameter is divided into twelve parts, 
called digits ; and when the eclipse is partial, we say, 
five or six digits are eclipsed, as the case may be. 

Although complete obscuration commences only 
when the moon enters the shadow e t e, yet a dim- 
ness takes place a little before ; for if we join a e 
and d e, and produce the lines to meet the orbit 
m m, then it will be readily seen that a consider- 
able part of the sun's light will be prevented from 
falling upon the moon, when it is between those 
lines and the earth's shadow. This space is called 
the penumbra. 




In this figure a total eclipse of the sun is repre- 
sented. 



ECLIPSES. 



77 



3i is the moon in her orbit, and exactly between 
the sun and earth, or in conjunction with the sun, 
and e is the earth. 

m a I m is the moon's shadow obscuring a small 
portion of the earth's surface. To the inhabitants 
between a and Z, the sun will appear totally 
eclipsed, because the lines a M and Z m extend tc 
the top and bottom of the sun's diameter a b. 

But to persons living between a and l, and I 
and d, the sun is still visible. 

If, however, a m, and Z m, do not reach to a and 
b, but only to the two points h and f, then a partial 
eclipse will take place to some persons near a and 
Z, but at the centre c, there will be a total eclipse. 
Should the shadow of the moon not reach the 
earth at all, then the moon will cross the sun's disk 
and appear like a black spot upon it, and a ring of 
the sun's disk will be visible ; this is called an an- 
nular eclipse, but this appearance is only presented 
to those inhabitants near the axis of the cone. 

An annular eclipse is represented in this figure. 




78 OUTLINES OF ASTRONOMY. 

Here t is the extremity of the shadow, not 
reaching the earth at a; join a m, a wi, produce 
these lines to h and f. 

Then that part of the sun's disk which is be- 
tween h and f will be eclipsed, but a h, and f b, 
will be visible to a spectator at a, who will see a 
rim of light of which the breadth is a h. 




The reason why partial eclipses of the sun take 
place is, that it seldom happens that the sun, earth, 
and moon have their centres exactly in the same 
line — and the infrequency of eclipses is caused by 
the orbit of the moon not being coincident with 
that of the earth. 

A total eclipse of the sun, April 22, 1715, was 
seen by most of the inhabitants of the south of 
England. 

Dr. Halley, a celebrated astronomer, and the 
inventor of the sextant, so valuable to sailors, has 



ECLIPSES. 79 

given a most interesting account of this event in 
the Philosophical Transactions. This eclipse was 
a very remarkable one ; during the total darkness 
(which lasted in London for three minutes twenty- 
three seconds), the planets Jupiter, Mercury, and 
Venus, were seen, and also some of the fixed stars. 
Annular eclipses are also very rare : one was ob- 
served in Scotland in 1737. This eclipse was an- 
nular at Edinburgh for five minutes forty-eight 
seconds. 

The beginning and end of solar eclipses can be 
observed with great accuracy, and are sometimes 
used in determining the longitude of places on the 
earth's surface. 

QUESTIONS. 

What is meant by the Phases of the moon ? Which way 
is the crescent of the moon always turned ? 

What is meant by the word Eclipse ? How only can an 
eclipse of the moon happen ? At what period of the moon 
do eclipses happen ? Why is she not eclipsed every month ? 
What two kinds of eclipses of the moon are there ? 

When does an eclipse of the sun happen ? What great 
difference exists between lunar and solar eclipses ? What 
gives us a good representation of a total eclipse of the sun ? 

What number of eclipses can happen in one year ? 
How many of these may be solar ? How many lunar ? 
Into how many parts is the moon's diameter divided for 
measurement ? What are these parts called ? How do we 
express the extent of a partial eclipse ? What is an annu- 
Jar eclipse ? 



SECTION X. 
TIDES. 

The moon is also the principal cause of the 
tides. 

To explain this, let us suppose that the whole 
earth is covered with water ; then, if there was 
no attraction arising from any external body, the 
water would gird the sphere of the earth, lying 
upon it like the rind of the orange, but always at 
the same depth. 

But the moon attracts the earth, and by this at- 
traction draws under her the waters of the earth, 
so that the waters stand indeed upon a heap under 
her disk. As the earth revolves round her axis, 
successive portions of her surface pass under the 
moon, and these places have successively high 
water. 

Six hours after the high tide the waters are at 
the lowest ebb, and in six hours more, we have 
high tide again ; or, referring to the figure, if b is 
the place of high tide when the moon is visible, b 
is the place of high tide when she is invisible, or 
the moon produces a tide on opposite sides of the 
earth at the same time. To explain this pheno- 

(80) 



TIDES. 



81 




menon, let the figure a l a I represent the waters 
of the ocean bounding the earth ; then it is clear 
that the water at a will be higher than any other 
part of the waters, because it is nearest the moon, 
and the force of attraction will be greatest there. 

Now let us look at the opposite hemisphere : 
there it is also clear, that the point b is further 
from the moon than any other point c near to b; 
thence the attraction at b, will be less than the at- 
traction at c, or the waters above &, will be less 
drawn to the moon than the waters above c, and 
they will consequently stand higher at b, than at 
any other point in the hemisphere l b I — or in 
other words, we have high tide at a. It may be 
remarked that the sun is also an active agent in 
producing the tides. 

When the moon and sun both act in the same 
direction upon the waters, the tides are the largest; 
this happens at new and full noon, and the tides 



82 OUTLINES OF ASTRONOMY. 

are called spring tides ; but when they act in con- 
trary directions, the tides are called neap, and are 
then the lowest ; this happens at the end of the 
first and beginning of the third quarter of the 
moon. 

In the preceding illustration of the cause of the 
tides, it is assumed that the place of high water is 
directly under the moon ; this, however, is not the 
case. For, although the waters directly under the 
moon are drawn towards it with greater force than 
any other part of the ocean, yet the water will not 
have reached its greatest height at any particular 
place till some hours after the moon has passed its 
meridian. 

For the waters attracted to the moon, when at 
this particular spot, will, from the impulse given, 
continue to flow to it, and rise there, until the moon, 
overcoming this impulse by a new one, shall draw 
them to some other spot. 

Thus, on a summer's day, the intensity of the 
sun's rays is greatest at twelve o'clock ; but the 
hottest part of the day is between two and three, 
for up to that time the actual stock of heat con- 
tinues to increase, and the quantity received from 
the sun exceeds that lost by the earth. Thus also 
the heat of July and August always exceeds the 
heat of June ; although in the latter month the days 



TIDES. 83 

are the longest, the heat of the sun is greatest, and 
that body is at its greatest elevation. For the heat 
on the earth will increase, so long as the radiation 
of heat from the earth during the night is less than 
the quantity added during the day. \ 

The word neap is said to be derived from a 
Latin word inops, signifying scanty ; thus neap 
tides are tides scanty of water. Others say, from 
an ebb, whence nebb, and so neap. 

QUESTIONS. 

What is the principal cause of the tides ? How are they 
produced ? How often does the tide rise and fall ? What 
is another active agent in producing tides ? 

What are the highest tides called ? When do they hap- 
pen ? What are the lowest called ? At what time do they 
take place ? 

When are the tides in reality at their greatest height ? 
What is the derivation of the word neap ? What is its 
meaning ? 




The Solar Svstem. 



SECTION XL 
THE SOLAR SYSTEM. 

The ancients believed that the earth is the 
centre of the universe, and that the sun is a com- 
paratively small body, and, with the moon and 
stars, revolves round our globe. Pythagoras, in- 
deed, and a few others, imagined a system much 
nearer the truth, but which was not generally 

(84) 



THE SOLAR SYSTEM. 85 

adopted till within a few centuries. Even so late 
as in the early part of the seventeenth century, 
the illustrious Galileo was persecuted for dissenting 
from the old opinion. Yet one of the most reason- 
able systems of antiquity, that of Ptolemy, was 
so beset with difficulties, that Alphonso X., who 
succseded to the throne of Leon and Castile, in 
1253, and was the most learned prince of the 
time, said, when considering it, " Were the uni- 
verse thus constructed, I could have, given the 
Deity some good advice, if he had called me to 
his councils at its creation." This has frequently 
been quoted as an impious remark, and certainly 
it is not a form of expression to be imitated ; but 
its severity was only intended for the system then 
taught, and not for the plan of creation. 

The system which is now received, was revived 
and much improved by Nicholas Copernicus, who 
was born at Thorn, in Prussia, in 1473. He had 
the work containing his system, prepared in 1530; 
but, from dread of opposition, he did not venture 
to publish it till 1542. He received a printed copy 
of it just before his death, which took place on the 
23d of May, in that year. 

According to this system, the earth and all the 
other planets revolve round the sun in their vari- 
ous orbits. 
8 



86 OUTLINES OF ASTRONOMY. 

The ancients were only acquainted with five 
planets ; but as we know that the earth is a planet, 
and have discovered five others, we are certain of 
the existence of eleven. 

Besides these there are eighteen satellites, or 
secondary planets, and many comets. All these 
bodies together form the Solar System. We shall 
treat of them in their order. 

To show in what a different light modern science 
has placed the universe from that in which the an- 
ciente viewed it, we need only state, that though 
several planets are much larger than the earth, 
and though the sun is more than five hundred 
times larger than all the planets and other bodies 
which revolve round it, yet the whole solar sys- 
tem, compared with all the stars whose existence 
is known to us, is no greater, than is one indivi- 
dual compared with the whole population of our 
globe. When some one inquired of Anaxagoras, 
who was born five hundred years before Christ, 
for what purpose he was made, he replied, " To 
contemplate the stars." With what sublime feel- 
ings would he contemplate them now ! 

QUESTIONS. 

What was the opinion of the ancients in regard to the 
universe ? What occurred in the beginning of the 17th 
century ? Who revived and much improved the system 



THE SOLAR SYSTEM. 87 

now taught ? Where and when was he born ? What is 
the principal feature of his system ? 

How many planets were known to the ancients ? With 
how many are we acquainted ? How many satellites are 
there ? What other bodies are there ? What do they 
together constitute ? 



SECTION XII. 
MERCURY. 

The planet which is nearest to the sun is called 
Mercury, and its distance from that luminary is 
about thirty-seven millions of miles. It makes a 
revolution round the sun in eighty-eight days, or 
about three months, which is therefore the length 
of its year ; and to perform this, it must move in 
its orbit at the rate of one hundred and twelve 
thousand miles an hour, or thirty-one miles in a 
second. This is swifter than the motion of any of 
the other planets. As its diameter is three thou- 
sand one hundred and forty miles, it is about fif- 
teen times smaller than the earth. 

Mercury is the densest of the planets. It is 
nearly as heavy as a globe of lead of its own dia- 
meter. 

It nray sometimes be seen, during a few minutes, 
in the morning and evening twilight, in the spring 
and autumn. At all other times the dazzling light 
of the sun conceals it from the naked eye. 

The sun at Mercury appears seven times larger 
than to us, and sheds seven times more light there. 

Mercury performs a revolution on its axis in 

(88) 



MERCURY. 89 



The Sun as seen from Mercury. | ^the S 



twenty-four hours and five minutes; its day is 
therefore nearly of an equal length with ours. As 
it is an inferior planet, or is nearer the sun than 
the earth is, it sometimes passes directly between 
us and the sun ; and it then looks like a dark spot 
moving over the sun's surface. This is called a 
Transit of Mercury. The last transit took place 
on the 8th of May, 1845. The next two will 
occur on the 9th of November, 1848, and the 12th 
of November, 1861. The reason why these do 
not occur in every revolution of the planet round 
the sun, is, that it generally passes above or below 
that body, or, in other words, the plane of its orbit 
is not coincident with the plane of the earth's orbit. 
When examined with the telescope, it is seen to 
8* 



90 OUTLINES OF ASTRONOMY. 

have phases, like the moon ; and several very high 
mountains are supposed to have been discovered 
on its surface. 

QUESTIONS. 

What planet is nearest the sun ? What is its distance ? 
In what time does it make a revolution ? What is its rate 
of motion ? What is its diameter ? How much smaller is 
it than the earth ? What is its density ? What is the ap» 
pearance of the sun from Mercury ? 

In what time does it revolve on its axis ? What is a 
transit of Mercury ? When did the last occur ? When 
will the next take place ? What may be observed with the 
telescope in regard to Mercury ? 



SECTION XIII. 

VENUS. 

The planet which is nearest to the sun after 
Mercury, is Venus. It moves round that body in 
two hundred and twenty-five days, at the distance 
of 68,000,000 of miles from it, and therefore at 
the rate of 80,000 miles an hour. It makes 
a revolution on its axis in twenty-three and a 
half hours ; and it receives about twice as much 
light from the sun as we do. Its diameter is 7,800 
miles. 

Venus comes nearer to the earth than any other 
body except the moon. The nearest part of its 
orbit is about 27,000,000, and the farthest about 
163,000,000 of miles from us. It has phases 
like the moon, but we never see it when full, be- 
cause then the sun is between it and the earth. It 
is to us the brightest of all the stars, and if we 
could see its bright side when it is in the nearest 
part of its orbit, it would appear twenty-five times 
larger than it does. 

Mountains of great elevation have been disco 
vered in this planet. One of them is thought to be 
twenty-two miles in height, or five times as high 

(91) 



92 OUTLINES OF ASTRONOMY. 

as the highest on our globe. Some have supposed 
that they discovered a moon accompanying Venus, 
but this yet remains to be ascertained. There ar*> 
transits of this planet, as there are of Mercury, but 
they occur very rarely, only twice in about one 
hundred and twenty years. There will be one in 
1874, and another in 1882. They are of great 
use to astronomers, especially in determining the 
distance of the sun. 

This planet is a great favorite with the poets. 
It has generally been cajled by them Phosphorus 
or Lucifer when it is the morning star, and Hes- 
perus or Vesper when it is the evening star. 

QUESTIONS. 

What is the next planet in the order of distance from the 
sun ? In what time does it revolve round that body ! 
What is its distance ? What is its rate of motion ? In 
what time does it revolve on its axis ? What is its diame- 
ter ? 

What is the distance between Venus and the earth when 
they are nearest ? When they are most distant ? In 
what does it resemble the moon ? Why do we never see 
it full ? What is its comparative brightness ? If we saw 
its bright side when nearest, how large would it appear ? 

How high is one of the mountains in Venus supposed to 
be ? How often do transits of this planet occur? When 
will the next two take place ? In what are they especially 
useful to the astronomer ? 

What do the poets call Venus when it is a morning star ? 
What when it is an evening star ? 



SECTION XIV. 
THE EARTH. 

The earth is the third planet in the order of 
distance from the sun. 

As we have already treated of its figure and 
magnitude, and of its satellite, the moon, we need 
not say much of it in this place. 

The earth moves round the sun at a mean dis- 
tance of ninety-five millions of miles, in three hun- 
dred and sixty-five days five hours forty-eight 
minutes and forty-nine seconds, and therefore at 
the rate of sixty-eight thousand miles an hour. 
Its orbit is an ellipse, of which the sun occupies a 
place somewhat removed from the centre. Thence 
we are nearer to the sun at one period of the year 
than at another, by a difference of three millions 
of miles. It may seem strange that we are near- 
est in midwinter ; but the obliquity of the sun's 
rays at that season, will account for their diminished 
intensity. 

As a short pendulum moves more rapidly than 
a long one, so a planet that is near the sun proceeds 
faster in its orbit than one which is more remote. 
Hence, though the earth moves at tin rate of 69,600 

(93) 



94 OUTLINES OF ASTRONOMY. 

miles an hour in January, it only moves 66,400 an 
hour in July, which makes a difference in each hour 
of three thousand two hundred miles. Its motion 
round the sun is from west to east ; and as it also 
turns round each day on its axis while proceeding 
in its orbit, its inhabitants on or near the equator 
are not only carried forward with the speed already 
mentioned, but are also carried round on its surface 
at the rate of 1040 miles an hour. 

These statements may at first seem incredible ; 
but they will appear quite natural, when the learn- 
er comes to calculate for himself the motions and 
magnitudes of the planets and other heavenly 
bodies. 

QUESTIONS. 

Which planet is the earth in the order of distance from 
the sun ? What is its distance ? In what time does it 
make a revolution in its orbit ? At what rate does it 
move ? What is the shape of its orbit ? How much 
nearer to the sun are we at one season than at another ? 
When are we nearest? Why are the sun's rays less in- 
tense at that time than in summer ? When does the 
earth move fastest ? When slowest ? Wb^t is the 
difference in speed ? In what direction does it move 
round the sun ? With what velocity does its surface near 
the equator move ? 



SECTION XV. 
MARS. 

We now come to those planets which are called 
superior, because they revolve round the sun 
beyond the orbit of the earth ; as those are called 
inferior which are nearer to the sun than our globe. 

The first of these is Mars, which moves in its 
orbit at the rate of fifty-five thousand miles an hour, 
at the distance of 145,000,000 of miles from the 
sun, and accomplishes a revolution in 687 of our 
days. Its own day is not very different in length 
from ours, since it revolves on its axis in twenty- 
four hours thirty-nine minutes and twenty-one 
seconds. Its diameter is about forty-two hundred 
miles, or little more than half that of the earth. 

The appearance of this planet is remarkable for 
its redness. When it is nearest to the earth it is only 
50,000,000 of miles from us, and it is then a very 
beautiful object, equalling with its ruddy beams 
the splendor of the brightest stars. But when it is 
at its greatest distance it is 240,000,000 of miles 
from us, and is then scarcely visible, appearing 
twenty-five times smaller than when it is nearest. 

Like Venus, Mars has a density somewhat less 

(95) 



96 OUTLINES OF ASTRONOMY. 

than that of the earth, which is about five and a 
half times that of water. 

When seen through the telescope, the surface of 
Mars exhibits spots or belts, which vary greatly ■ 
in their appearance at different times. It has been 
thought that some of these changes may be caused 
by the accumulation and subsequent thawing of 
snow and ice ; and as the spots seem to increase 
in the winters of that planet, the opinion may be 
correct. 

Sir J. Herschel thinks he can discern in Mars* 
with perfect distinctness, the outlines of what may 
be seas and continents ; and that the ruddy coloc 
which distinguishes its light, indicates an ochrej 
tinge in the soil, which, though in a stronger de 
gree, gives it the appearance to us, which our reO 
sandstone districts may have to the inhabitants of 
Mars. But others ascribe this color to the density 
of the atmosphere of Mars, which might affect the 
rays of light as they are affected by our atmo- 
sphere when they pass through it horizontally in 
the morning or evening, 

QUESTIONS. 

What are inferior planets ? What superior ? Which iz 
the first of the superior ? At what rate does it move m its 
orbit ? At what distance from the sun ? In what time 
does it maKe an annual revolution ? In what time a diurnal 
revolution fc What is its diameter ? 



MARS. 97 

What is remnrkao.e in its appearance ? What is its dis- 
tance from us when nearest ? When farthest ? How much 
smaller does it seem when farthest than when nearest ? 
What is its density ? What does its surface exhibit when 
seen through the telescope ? How are these accounted 
tor I What did Sir John Herschei think he could discover 
on Mars ? To what do others ascribe the colour of the at- 
mosphere of Mars ? 



SECTION XVI. 
THE ASTEROIDS. 

The planets which we have mentioned will be 
found, on examination, to be placed at remarkably 
regular invervals from the sun ; and this regu- 
larity holds also in regard to the three outer pla- 
nets, Jupiter, Saturn, and Herschel. But between 
Mars and Jupiter, the distance is so great that the 
order seemed to be interrupted there. This led to 
the suspicion that there might be in this space a 
yet undiscovered planet ; and the result of the sus- 
picion was most wonderful : namely, the discovery, 
in little more than six years, of four planets, which 
had been revolving in their places during unknown 
ages, but had never before been detected by human 
vision. 

These planets are much smaller than any of 
the others, and are, from several circumstances, 
supposed to be the fragments of a larger body, 
which has been separated by some convulsion or 
concussion. 

They are called Vesta, Juno, Ceres, and Pallas. 

Vesta was discovered on the 29th of March, 
1807, by Dr. Olbers, of Bremen, in Germany. 

(98) 



THE ASTEROIDS. 99 

It is 225,000,000 of miles from the sun, and com- 
pletes its revolution in three years seven months 
and a half. Its diameter is estimated at two hun- 
dred and seventy miles ; according to which its 
surface is of less extent than Great Britain, Ire- 
land, and France. It is the smallest heavenly 
body with which we are acquainted. 

Juno was discovered by Mr. Harding, at Lilien- 
thal, near Bremen, on the 1st of September, 1804. 
Its distance from the sun is 252,000,000 of miles, 
and its revolution round that luminary is com- 
pleted in four years and four months. Its diameter 
has recently been estimated at 1425 miles, though 
other calculations make it much smaller. 

Ceres was discovered by Mr. Piazzi, of Palermo, 
in Sicily, on the 1st of January, 1801, that is, on 
the first day of this century. Its distance from 
the sun is 263,000,000 of miles, which is nearly 
three times as far as the distance of the earth from 
that body. It runs through its orbit in four years 
seven months and ten days. Its diameter is esti- 
mated at 1624 miles, and it seems to have a very 
high and dense atmosphere. 

Pallas was discovered on the 28th of March, 
1802, by Dr. Oibers, the same astronomer who 
discovered Vesta. It, in some respects, resembles 
Ceres. It is equally distant from the sun, and its 



100 OUTLINES OF ASTRONOMY. 

year is only about one day shorter. It has a 
somewhat less ruddy color than that planet, and 
its atmosphere is not quite so high. The changes 
which may be observed in its appearance are like 
what would result from the occasional prevalence 
of fogs or clouds in its atmosphere. Its diameter 
is 2099 miles, and it is therefore nearly of the 
same size as the moon. 

These little bodies, compared with the other 
planets, are like islands among continents ; and, 
since they are so small, and never come near us, 
it is difficult to become accurately acquainted with 
them. Their diurnal revolutions have not been 
determined ; and, if they are the fragments of a 
larger body, we have not yet the means of learn- 
ing what effect the separating shock had upon 
them. 

A new planet was discovered by Professor 
Hencke, of Dresden, in Saxony, on the 8th of 
December, 1845, at eight o'clock in the evening. 
It is supposed to be one of the asteroids. The 
elements of its orbit have been calculated by Pro- 
fessor Encke, of Berlin. The discoverer left the 
name of the planet to be determined by his illus- 
trious friend, Mr. Encke, who has called it As- 

TR^A. 

Mr. Darby, of Washington, has calculated that 



THE ASTEROIDS. 101 

its distance from the sun is 250,000,000 miles, 
and that it revolves between the orbits of Juno and 
Vesta. 

QUESTIONS. 

What led to the suspicion that there might be a planet 
between Mars and Jupiter ? What was the result of this 
suspicion ? 

What are these planets supposed to be ? t What are their 
names ? 

When was Vesta discovered ? By whom ? How far is 
it from the sun ? In what time does it make its annual 
revolution? What is its diameter? What then must be 
its surface ? How does it rank in size among the heavenly 
bodies ? 

When and by whom was Juno discovered ? What is its 
distance from the sun ? What the time of its annual revo- 
lution ? What is its diameter ? 

By whom was Ceres discovered? When ? What is its 
distance from the sun ? What the time of its annual revo- 
lution ? What its diameter ? What kind of an atmosphere 
does it seem to have ? 

When was Pallas discovered? By whom ? How does 
it differ from Ceres ? To what might the changes in its 
appearance be ascribed ? W T hat is its diameter ? What 
body does it resemble in magnitude ? 
9* 



SECTION XVII. 
JUPITER. 

We now come to those planets, which, with 
their satellites, present a sort of miniature repre- 
sentation of the solar system. Of these the first 
is Jupiter. 

This planet may generally be distinguished by 
its great size and brightness ; for, except Venus, 
in has no rival among the stars. 

Its distance from the sun is 495,000,000 of 
miles ; and though it moves at the rate of nearly 
thirty thousand miles an hour, it occupies almost 
twelve years in travelling through its orbit. 

Its diameter is about ninety thousand miles. It 
is therefore more than thirteen thousand times 
larger than the earth. Yet it makes a revolution 
on its axis in nine hours and fifty-six minutes. 
The inhabitants on its equator would therefore be 
carried round at the rate of 27,000 miles an hour. 
This makes its days and nights only five hours 
each ; and hence the sun and stars must, from its 
surface, appear to move through the sky with much 
greater rapidity than from that of the earth. The 
number of its days and nights in a y^ar is 10,471. 

(102) 



JUPITER. 103 

its equatorial diameter is six thousand miles greater 
than its polar, and this would naturally result from 
its rapid motion on its axis. 

Jupiter, when observed with a telescope, is al- 
ways found to be marked with stripes, which are 
called its belts. They are usually three or four 
in number ; but their number and appearance un- 
dergo changes from time to time. They are 
usually supposed to be clouds, drawn into the shape 
which they assume by the rapidity of the planet's 
rotary motion. Mr. Dick, however, entertains the 
plausible opinion, that the dark stripes are portions 
of the globe, which are visible, and the bright parts, 
a sort of luminous clouds. 




Owing to Us great distance, the sun must appear 



104 OUTLINES OF ASTRONOMY. 

much smaller as seen from Jupiter, than it appears 
to us. But as some compensation, the planet has 
four satellites or moons, which are not very dif- 
ferent in size from our moon, and move round that 
body in two, three, seven, and sixteen days re- 
spectively. As these bodies, in their rapid move- 
ments, pass and eclipse each other, they must add 
greatly to the beauty and interest of the firmament 
in which they revolve. 

QUESTIONS. 

To what might the rest of the planets with their satel- 
lites be compared? Which is the first? Which is it in 
the order of distance from the sun ? What is its distance ? 
At what rate does it move in its orbit ? How long is its 
year ? 

What is its diameter ? How much larger is it than the 
earth ? In what time does it revolve on its axis ? How 
fast are the inhabitants on its equator carried forward ? 
What is the length of its days and nights ? How many 
days and nights are in its year ? How much greater is its 
equatorial than its polar diameter ? What maybe supposed 
to have caused this difference ? 

What may be observed through the telescope on the sur- 
face of Jupiter ? What are these usually supposed to be ? 
What is Mr. Dick's opinion ? 

How many satellites has Jupiter ? What is their size ? 
In what time do they revolve round their primary ? 



SECTION XV111. 
SATURN. 

This is the most wonderful body, with which 
we are acquainted, in the solar system. Its dis- 
tance from the sun is 900,000,000 of miles, and 
its diameter is 79,000. It travels at the rate of 
21,000 miles an hour. This velocity is not easily 
conceived, yet the planet occupies nearly four hours 
in passing the length of its own diameter. It 
makes a revolution round the sun in twenty-nine 
and a half of our years ; so that few persons on 
this globe live through three of Saturn's years. 

As there are twelve signs or constellations in 
the zodiac, Saturn requires nearly two years and 
a half to pass through each. When its place, 
therefore, is once known, it can easily be traced 
at any time afterwards. 

Though Saturn is one thousand times larger 
than the earth, it revolves on its axis in ten hours 
and a half. Its polar diameter might, therefore, 
be expected to be less than its equatorial ; and they 
are in the proportion of eleven to twelve. 

It is calculated that the light which Saturn re- 
ceives from the sun, is ninety times less than 

(105) 



106 OUTLINES OF ASTRONOMY. 

that afforded to the earth ; but, as some compen- 
sation for the scantiness of this supply, it is sur- 
rounded by two magnificent rings, and seven 
moons, which must give its nightly sky a bright- 
ness and beauty which would almost bewilder us. 

The interior ring is 21,000 miles from the pla- 
net, and is 20,000 miles broad. The exterior ring 
is twelve hundred miles in breadth, and the in- 
terval between them is about two thousand. They 
are supposed to be one hundred miles in thickness ; 
and, with all the moons except the fifth, they are 
in the same plane with the equator of the globe 
round which they revolve. The rings make a re- 
volution in the same time as the planet. Some of 
the moons appear much larger than ours. The 
nearest of them makes a revolution in twenty-two 
hours, and the farthest in seventy-nine days. The 
first is 20,000 miles beyond the external ring, and 
the other more than 2,000,000. The rest are 
placed between these at irregular distances, and 
revolve at irregular intervals. 

We have much reason to believe that the satel- 
lites and rings, as well as the planet itself, are in- 
habited, and that they contain a population several 
thousand times more numerous than our globe. 
They may also be superior to us in stature and 
capacity. The uninstructed are prone to consider 



SATURN. 107 

this world the centre of the universe, and our race 
as the only people in creation. But it might teach 
our pride a useful lesson, to reflect on the idea 
which the inhabitants of Saturn must form of our 
globe, and indeed of all the other planets of the 
solar system, when they compare them with their 
own glorious home. 

QUESTIONS. 

Which planet is Saturn in the order of distance from the 
sun ? What is its distance ? What its diameter ? At 
what rate does it travel ? In what time does it complete 
its annual circuit ? How may its place in the zodiac be 
traced ? 

How many times larger than the earth is Saturn ? In 
what time does it revolve on its axis ? What is the pro- 
portion of its polar and equatorial diameters ? 

How many times less light does Saturn receive from the 
sun than the earth ? How is it compensated for this scanti- 
ness of supply ? What are the dimensions of the rings ? 
In what plane do they revolve ? In what time ? 

In what time does the nearest moon of Saturn revolve 
about its primary ? In what time the most distant ? What 
are their distances ? 



SECTION XIX. 
URANUS. 

From certain irregularities in the motions of the 
bodies in the solar system, which could not be 
otherwise satisfactorily accounted for, it was long 
suspected that there must be a planet still farther 
removed from the sun than Saturn ; and this 
idea proved to be correct. In 1781, Sir William 
Herschel discovered that a certain star exhibited 
too much motion to be a fixed star, and supposed 
it to be a comet. But it was soon found to be a 
planet ; and many call it Herschel, in honour of 
its discoverer. It had been seen by other astro- 
nomers at various times, but they did not suspect 
it of belonging to the solar system. 

This most remote of the planets with which we 
are acquainted, is 1,800,000,000 of miles from the 
sun. Though its motion in its orbit is slower than 
that of any other body with which we are ac- 
quainted, yet it moves at the rate of fifteen thou- 
sand miles an hour. It requires eighty-four years 
to make one revolution round the sun ; and has, 
therefore, described little more than three-fourths 

(108) 



URANUS. 109 

of ihis mighty circle since it first became known 
as belonging to our system. 

Its diameter is 35,000 miles, which makes it 
more than eighty times larger than the earth. It 
no doubt revolves on its axis like all the other pla- 
nets, but this our telescopes have not yet enabled 
us to ascertain. It is attended by six satellites, two 
of which frere discovered by Dr. Herschel, and the 
other four by his sister, Miss Caroline Herschel. 

The sun, to the inhabitants of this planet, must 
appear three hundred and sixty times smaller than 
to us, and consequently we cannot suppose that 
they receive more than one three hundred and six- 
tieth as much light as we do ; yet it is calculated 
that, even at that immense distance, the sun would 
shed more light than eight hundred full moons. 
It has been supposed that Uranus cannot be in- 
habited on account of its being so far removed 
from the source of heat. But the degree of heat 
of a world, perhaps, depends more upon the nature 
of its atmosphere than on its nearness to the sun. 

OTHER PLANETS. 1 

In the distribution of planets around the sun, 
as well as in that of satellites around their pri- 
maries, there is considerable regularity observed. 
10 



110 OUTLINES OF ASTRONOMY. 

The largest bodies generally have a central posi- 
tion ; but Uranus, compared with those planets 
which are nearest the sun, is very large. It may, 
therefore, be considered not improbable that there 
are other planets still more remote. This does 
not seem more incredible now than it once ap- 
peared that Uranus should be discovered. Indeed, 
it would be strange if the solar system were more 
fully explored than this comparatively diminutive 
world in which we live ; and yet the navigator is 
still discovering new islands and even continents. 

QUESTIONS. 

From what was it suspected that there was a planet be- 
yond Saturn ? When was it discovered ? By whom ? 
What is it sometimes called ? 

Which planet is Uranus in the order of distance from the 
sun ? How distant is it ? At what rate does it move in its 
orbit ? How long is its year ? How much of its orbit has 
it described since it became known to the world ? 

What is its diameter ? How many times larger is it 
than the earth? How many satellites has it? By whom 
were they discovered ? 

How many times smaller must the sun appear to the in- 
habitants of Uranus than to us ? To the light of how many 
full moons would this be equal ? 

Why have some supposed that Uranus cannot be inhabit- 
ed ? What reply may be made to this ? 



SECTION XX. 
COMETS. 

We must now consider certain members of that 
great collection of wonders which we call the solar 
system, which are, if possible, still more extraordi- 
nary than even the planets. These are the Comets, 
of which, according to that great astronomer Kepler, 
there are more in the depths of space, than there 
are fishes in the bosom of the ocean. 

Very many comets have been seen during late 
years, and we have no reason to suppose that they 
were less numerous at any former period. But 
the telescope enables us to observe many which 
could not be seen without it, and to observe those 
which are visible to the naked eye, much better 
than we could otherwise. 

The Greek and Roman writers now and then 
mention these bodies, and sometimes give the place 
in the heavens where one appeared. But they 
knew less about them than we do. Some of them 
believed that they are distant from the earth, 
while others, especially Aristotle and his followers, 
thought that they are vapors formed in our atmo- 
sphere. But it can easily be proved that this is 

(111) 



112 OUTLINES OF ASTRONOMY. 

entirely impossible. For when they are seen at 
the same moment from two distant points, for in- 
stance from Washington and Paris, they appear 
at each place, to be in the same region of the 
heavens. If they were not very distant, they would 
appear eastward from the one place, when they 
appear westward from the other. 

It is worthy of notice that, though the Greeks 
and Romans have left us little information of im- 
portance respecting these bodies, the Chinese litera- 
ture contains accurate records of the course of 
some which appeared more than five hundred years 
before the Christian era. 

There are three parts of the comet distinguished ; 
the nucleus, the envelope, and the tail. The nu- 
cleus is the head or dense part of the comet ; the 
envelope, also called the coma, from its resem- 
blance to hair, and therefore called the brush by 
the Chinese, is a sort of atmosphere, of which the 
tail seems to be an extension. Sonetimes there is no 
solid nucleus, but in its stead only a dense mass 
of vapor. 

The orbits of comets are elliptical, and have the 
sun in one of their foci. Those bodies are there- 
fore much nearer to that luminary at one period 
than at another. Some of them proceed so far 
out into space that they do not return for several 




(114) 



COMETS. 1 1 -3 

thousand years. A magnificent one appeared in 
the year 1680, which, according to the astronomer 
Encke, occupies 8800 years in performing one re- 
volution ; while another, whose orbit was determin- 
ed by the same astronomer, and which is therefore 
called Encke's Comet, performs its journey in three 
years and four months. 

Their tails, which generally extend in a direc- 
tion opposite from the sun, increase as they ap- 
proach that body, and decrease as they recede from 
it, till, to appearance at least, they vanish entirely. 
Some of these tails are of an immense length. 
That of a great comet which appeared in 1811, 
was calculated at 130,000,000 of miles ; and that 
of the beautiful comet of 1843, was supposed to 
be of equal length. 

These bodies, according to a universal law, con- 
stantly increase their speed as they approach the 
sun, and therefore move fastest at the perihelion. 
The comet of 1680, when at its perihelion, was 
within 146,000 miles of the sun's surface, and 
moved, according to the calculations of Newton, 
at the rate of 880,000 miles an hour. But others 
suppose that it moved much faster. 

It is quite possible that some of the comets are 
inhabited. If there are people on this one, how 
wonderful the sun must appear to them ! When 
they are near their aphelion, they see it for up in 



116 OUTLINES OF ASTRONOMY. 

the sky, scarcely distinguishable from the rest of 
the stars ; but as they approach it, it appears to 
descend towards them, and slowly but steadily in- 
crease in size. As it seems to come nearer to 
them, its increase becomes more and more rapid, 
and at last it fills the sky, almost to the horizon, 
on every side like a great canopy of fire, which, 
rushing across their heads, again recedes in a 
direction somewhat different from that in which it 
came. 

Comets were formerly viewed with terror, be- 
cause they were supposed to be the forerunners of 
great evils — of wars, famines, and plagues. Some- 
times whole nations have been thrown into con- 
sternation by the appearance of one of these bo- 
dies, because they supposed it would strike and de- 
stroy the earth. These ideas are still entertained 
to some extent by the uninstructed. But those 
who make themselves acquainted with the laws 
which govern the movements of the heavenly bo- 
dies, will learn that these fears are entirely ground- 
less, and will view the approach of these wanderers, 
trailing 

" In an inestimable length of light, 
Their golden train of tresses after them," 

as among the most interesting and beautiful phe- 
nomena of nature. 



COMETS. 1 1 7 



QUESTIONS. 

What did Kepler say in regard to the number of comets ? 
What writers of antiquity sometimes mention comets i 
What was the opinion of Aristotle and his followers re- 
specting them ? What proves this to be impossible ? In 
what literature are some of these bodies mentioned I How 
long ago did some of those mentioned appear ? 

What three parts of the comet are distinguished ? What 
is the nucleus ? What the envelope ? What the tail ? 

What is the shape of the orbits of comets ? How long 
are some of them absent in the distant parts of their orbits ? 
What is the period of revolution of the comet of 1680 ? In 
what period does Encke's Comet perform a revolution in its 
orbit ? 

What is the direction of their tails ? How do their tails 
change as comets approach and recede from the sun ? How 
long were the tails of the comets of 1811 and 1843 ? 

When do comets move fastest ? How near the sun was 
the comet of 1680 when in its perihelion ? At what rate 
did Newton suppose it to travel at that period ? 

What were comets formerly supposed to be ? What 
effect had their appearance at times ? What does astronomy 
teach us in regard to such fears ? 



SECTION XXI. 
METEORS. 

It may be proper to mention in this place an- 
other class of bodies as belonging to the solar sys- 
tem, though their relation to it is not yet ascer- 
tained. These are the Meteors, or shooting-stars, 
which must be inconceivably numerous, for they 
may be seen almost every night at every place over 
the whole globe. They generally dart towards the 
earth, and apparently vanish before they reach it. 
They mostly appear singly, but sometimes great 
showers of them continue to fail during several 
hours. It is said that fifty-two such are recorded 
by the Chinese ; one of them as having occurred 
six hundred and eighty-seven years before the 
Christian era. They are also mentioned, by the 
writers of other nations, as having occurred at 
various places ; but one of the most remarkable 
of which we have any knowledge, took place on 
the night of the 12th and 13th of November, 
1833. It extended over the whole of the United 
States and surrounding regions, and was one of the 
most wonderful spectacles that could be viewed. 

Sometimes meteors explode with a loud noise 

(118) 



METEORS. 119 

when they come into our atmosphere, and fall 
in fragments to the ground. Sometimes they dart 
through the air without falling to the ground, but 
pass off again into space. These are often of a 
huge size. One, which came within twenty-five 
miles of the earth, was supposed to weigh six hun- 
dred tons, and move at the rate of twenty miles 
in a second. 

When they are known to be solid, they are 
called Aerolites. These have sometimes, in their 
fall, set buildings on fire ; and it is recorded that 
they killed a monk, at Crema, in 1511; another 
at Milan, in 1650; and two Swedish sailors on 
shipboard, in 1674. 

They have often been analyzed, and are nearly 
always found to contain the same substances in the 
same proportions. These are silex, magnesia, 
sulphur, iron, nickel, and chrome. 

As a shower of meteors is seen nearly every 
year in some part of the world about the 12th of 
November, they are supposed to float in space 
through the part of the earth's orbit which it 
reaches at that period. Some think that they re- 
volve round the sun in immense numbers in an 
elliptical orbit, and that the earth, in passing near 
them at the period referred to, attracts them to its 
surface. Those which at various times appear 



120 OUTLINES OF ASTRONOMY. 

singly may also have been floating in space, and, 
coming near the earth, been drawn into its atmo- 
sphere. 

QUESTIONS. 

What proves the shooting stars to be very numerous ? 
How many meteoric showers are said to be recorded in the 
literature of the Chinese ? When did one of them occur ? 
When and where did one of the most remarkable take 
place ? How do meteors sometimes make their appear- 
ance ? How heavy was one which passed through the air 
supposed to be ? 

What are they called when known to be solid ? What 
circumstances are mentioned as having occurred from their 
fall ? Of what are they composed ? 

At what time of the year is there generally a meteoric 
shower seen in some parts of the world ? What is the 
theory in regard to their origin ? 



SECTION XXII. 
ZODIACAL LIGHT. 

There is a singular phenomenon sometimes 
visible immediately before sunrise or after sunset, 
in the place where the sun is about to rise or which 
he has just quitted on the horizon. It appears 
most conspicuously in February and March, when 
it is seen in the west, and, of course, in the evening, 
and in October and November, when it is seen in 
the east between dawn and sunrise. At some 
seasons, between the periods mentioned, it is seen 
on both sides of the sun, that is, before sunrise and 
after sunset. Its form is that of a pyramid with 
its base to the sun, rising above the horizon 
with its point generally directed to some part of 
the zodiac. 

Though the zodiacal light is often seen very dis- 
tinctly in Europe and this country, its appearance 
is much more striking near the equator, where the 
atmosphere is less misty and the twilight of shorter 
duration. " He who has lived many years in the 
zone of palms," says Humboldt, the eminent travel- 
ler, " retains a delightful recollection of the mild 
radiance with which the zodiacal light illumines 
11 (121) 



122 



OUTLINES OF ASTRONOMY. 




Zodiacal Light. 



a portion of the unvarying length of the tropical 
night. I have seen it occasionally more intensely 
luminous than the Milky Way in Sagittarius ; and 
that not only in the thin and dry atmosphere of 
the summits of the Andes, at the height of twelve 
or fourteen thousand feet above the level of the 
sea, but also in the boundless grassy plains of 
Venr zuela, as well as on the coasts of the ocean 
under the ever serene sky of Cumana. The phe- 



ZODIACAL LIGHT. 123 

nomenon was most peculiar when small fleecy 
clouds appeared projected upon the light, and stood 
out picturesquely from the luminous back-ground." 
Various explanations have been given of this 
phenomenon, but none has yet been agreed upon. 
Some have ascribed it to the atmosphere of the sun ; 
but Laplace has shown that, according to the laws 
of motion, that atmosphere cannot possibly be so 
large as we must suppose it to be if we would 
adopt his explanation. Some have ascribed it to 
a nebulous body which they suppose to float around 
the sun ; and others to a peculiarity of our own at- 
mosphere. A satisfactory account of this myste- 
rious appearance will probably be accompanied by 
some new and useful discoveries. 

QUESTIONS. 

At what time and where is the zodiacal light sometimes 
visible ? When is it seen in the west ? When in the 
east ? What is its form ? Where is its appearance much 
more striking than in our latitude ? What explanations of 
it have been offered ? 



SECTION XXIII. 
THE FIXED STARS. 

Having examined the solar system, we may 
now, in some measure, understand the constitution 
of the fixed stars ; for we have reason to believe 
that every one of them is a sun, with planets and 
comets revolving about it, but made invisible to us 
by distance. 

The number of fixed stars distinctly visible to 
us at once, on the darkest nights, seldom exceeds 
two thousand. 

They are distinguished from the planets, not 
only by preserving the same relative position, but 
also by a tremulous motion or twinkling in their 
light. 

The ancients, for the convenience of referring 
to the different stars, invented constellations. They 
imagined a number of persons, most of them con- 
nected with their mythology, and also of animals 
and inanimate things, to be drawn on the surface 
of the sky, and to include particular stars, or 
groups of stars. 

The stars, however, do not form the figure of 
the constellation, except in a few instances, where 

(124) 



THE FIXED STARS. 125 

there is some resemblance — the best and most 
s.rikinsj; is the constellation of the Great Bear. 
Some stars have particular names, as Sirius, Arc- 
turns, Regulus, &c. 

This method of dividing the heavens into groups 
of stars is very old — it is mentioned by Hesiod and 
Homer, the oldest heathen poets. Three of them 
are twice mentioned in the book of Job ; and in 
the prophecy of Amos, who lived eight hundred 
years before our Saviour, we read the following 
sublime passage : " Ye who turn judgment to worm- 
wood, and leave off righteousness in the earth, seek 
Him that maketh the seven stars and Orion, and 
turneth the shadow of death into the morning, and 
maketh the day dark with night : that calleth for 
the waters of the sea ; and poureth them out upon 
the face of the earth. The Lord is his name." 

The stars are also divided according to their ap- 
parent brightness into magnitudes. The brightest 
are of the first magnitude ; the next, less bright, 
are of the second magnitude ; and so on to the 
sixth, the least magnitude visible to the naked eye. 

There are eleven stars of the first magnitude 

visible to us who live in the northern hemisphere ; 

and six in the remaining half of the sky. There 

are about fifty of the second, and about one hun- 

11* 



126 OUTLINES OF ASTRONOMY. 

dred and twenty of the third magnitude visible 
to us. 

The milky way, or galaxy, may also be reckoned 
among the constellations ; for it is now known to 
be a collection of fixed stars, too small to be 
observed without telescopes of high magnifying 
power. Dr. Herschel observed five hundred and 
eighty-eight stars in the field of his telescope at the 
same time, and they continued equally numerous 
for a quarter of an hour. In one portion of the 
milky way he computed there were 250,000 stars. 

The telescope by which Dr. Herschel made his 
discoveries was an astonishing performance, when 




Herschel's Telescope. 



THE FIXED STARS. 127 

the labour and difficulty of adjusting its various 
parts are considered. Its length was forty feet, 
and the breadth of the aperture four feet. The 
fixed stars, when seen through it, have a bright- 
ness too great for the eye to bear. Dr. Herschel 
states that when the star Sirius was about to enter 
the field of view of the telescope, the light was 
equal to that of the approach of sunrise, and upon 
entering the telescope, the star appeared in all the 
splendor of the rising sun, and that it was im- 
possible to behold it without pain to the eye. The 
appearance of the stars seen through a telescope is 
different from that of the planets. The latter are 
magnified and show a visible body or disk ; but 
the former only shine with increased brightness, but 
with no disk. A proof of the vast distance of the 
stars. From observations it appears that consider- 
able changes have taken place among the fixed 
stars. Some have totally disappeared, and new 
ones have appeared. There are some stars which 
appear and disappear alternately ; others are found 
to vary with respect to magnitude and brilliancy, 
and many of the stars which appear single, are 
found, when examined with a good telescope, to 
be double or treble, and some even quadruple or 
quintuple. 

The discovery that the stars are frequently 



128 OUTLINES OF ASTRONOMY. 

double and multiple, is one of the grandest that 
was ever made in astronomy. Sir William Her- 
schel found seven hundred such ; but the recent 
improvements in the telescope have enabled several 
astronomers of great industry and talent, to in- 
crease the catalogue of these wonderful objects to 
about 6000, and to determine the periods in which 
many of them revolve about each other. 

Their periods of revolution are very various. 
Thus a double star in the constellation called the 
Crown has a period of 43 years, while that of one 
in the Swan is 452, that of one in the Lion 1200, 
and that of one in Bootes 1681 years. 

One of the most singular discoveries made in 
viewing the double stars, was that they are of dif- 
ferent colors. Thus in the constellation Hercules 
there is a double star, the larger one red and the 
smaller blue ; and a star in the Harp which ap- 
pears single to the unassisted vision, is found to 
consist of four stars, three of which are white, while 
the fourth is red. 

Several new stars have appeared in places 
where none were seen before. The appearance 
of one of these induced Hipparchus, who lived in 
the 2d century before Christ, to make a catalogue 
of the stars, so that if new ones appeared in future 
they might be detected. This was the first cata- 



THE FIXED STARS. 129 

logue of the stars, of which we have any account. 
Another new star was seen in 945, and one in 1264. 

But the first phenomenon of this kind of which 
we have a minute account, occurred in 1572. A 
new star was seen in Cassiopeia. It appeared sud- 
denly, and became more brilliant than Venus, so 
that it could be seen by some persons at noon-day. 
This wonderful sight caused the great Tycho 
Brahe to become an astronomer. It was of a daz- 
zling white with a bluish tinge, and its brightness 
caused the staff of Brahe to cast a shadow. It 
only retained its greatest brilliancy a few weeks, 
then diminished gradually, and at the end of six- 
teen months from its first appearance, could no 
longer be seen. 

It has been supposed that this star, and those 
which appeared in the tenth and thirteenth centu- 
ries, may have been but one, appearing periodically. 
If this is so it may be expected again about the 
year 1891. 

In 1604, a new star appeared in Serpentarius, 
and in 1670, another in the Swan. They both 
vanished after a short stay, and are not known to 
have made their appearance again. 

There are other stars which undergo periodical 
changes, appearing at one time of a greater, at 
another of a less magnitude. The lengths of their 



130 OUTLINES OF ASTRONOMY. 

periods are very different. Thus the star Algol 
in the constellation Perseus goes through its changes 
in two days twenty hours and forty-nine minutes, 
while a star sometimes called Stella Mira, in the 
Whale, has a period of 331 days, and another in 
the Swan is supposed to have a period of eighteen 
years. These are called variable stars. 

One of the most remarkable circumstances re- 
lating to the fixed stars is their immense distance. 
This may be illustrated in the following manner. 
If we look down a straight road, the pathways 
on each side seem to unite in the distance, and, if 
at this point there are two trees, one on each side 
of the road, they will appear but as one tree ; but, 
as we walk along, the trees gradually separate, 
and we see the road beyond them. Now the earth 
at one period of its revolution is 190,000,000 of 
miles nearer to some of the fixed stars than at a 
period six months before ) yet this enormous space 
makes no difference in the apparent distance of 
those stars from each other. 

The constellations to which we have referred 
are divided into three principal classes : those on 
the northern side of the equator are called northern 
constellations, and those on the opposite, southern 
constellations ; while those which are situated about 
that part of the celestial hemisphere, where the 



TIIE C0NSTELLATI0X5. 



131 



principal planets move, and which forms a belt or 
zone crossing the equator at two points, are called 
zodiacal constellations. 

Zodiacal is derived from a Greek word, signify- 
ing animal, since most of the constellations of the 
zodiac are named from animals. 



CATALOGUE OF THE CONSTELLATIONS. 

TWELVE ZODIACAL. 



NAMES. 

Aries . . the Ram . . . 


PRINCIPAL STARS. 

Arietis 


MAGN. 


Taurus . . 


the Bull. . . . 


Aldebaran . . 


. 1 


Gemini . . 


the Twins . . . 


Castor and Pollux 


1—2 


Cancer . . 


the Crab 






Leo . . . 


the Lion . . . 


Regulus . . . 


. 1 


Virgo . . 
Libra . . 


the Virgin . . . 
the Scales 


Spica Virginis . 


. 1 


Scorpio . . 
Sagittarius . 
Capricornus 


the Scorpion . . 
the Archer 
! the Goat 


Antares . . . 


. 1 


Aquarius . 


the Water Bearer 






Pisces . . 


the Fishes 







The northern constellations are thirty-four n 
number: the ancient and principal ones are mer* 
tioned in the annexed catalogue. 



PRINCIPAL STARS. 



Ursa Major . the Greater Bear 
Ursa Minor . the Lesser Bear 



Dubhe . . 1 
Pole Star . 2 



132 OUTLINES OF ASTRONOMY. 

NAMES. PRINCIPAL STARS. MAGN. 

Perseus Algenib . . 2 

Auriga . . the Wagonner . . . Capella . . 

Bootes Arcturus 

Draco . . . the Dragon 
Triangulum . the Triangle 
Corona Bo- cthe Northern 

realis \ Crown 

Serpens . . the Serpent 
Hercules . . with the Branch 

Lyra . . .the Harp Vega ... 1 

Sagitta . . the Arrow 

Aquila . . the Eagle .... Altair . . 1 

Delphinus . the Dolphin 

Cygnus . . the Swan Deneb Adige 1 

Cassiopeia . the Lady in her 

Chair 
Pegasus 
Andromeda 

There are forty-five constellations in the South- 
ern Hemisphere, the principal of which are the 
following : — 

Eridanus . . the River Po . . . Achernar . 1 
Cetus . . . the Whale 

Noah's Dove Phaet. . . 2 

Orion Betelguese . 1 

Argo Navis . the Ship Argo 

Canis Major, the Greater Dog . . Sirius . . 1 

Canis Minor the Less Dog . . Procyon . . 1 

Hydra Cor Hydrae . 1 

PicisAustralis,or the Southern Fish Fomalhaut . 1 



133 



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134 



OUTLINES OF ASTRONOMY. 



When astronomers wish to designate any of the 
stars in a constellation, they apply to them the 
letters of the Greek alphabet ; the first letter de- 
noting the brightest star ; the second letter, the 
star second in brightness, and so on. When they 
have gone through with this alphabet, they employ 
the Roman, and after that, the numerals. When, 
therefore, we see a star mentioned as a Leonis, 
we know that it is the principal star of the Lion ; 
when it is mentioned as j3 Lyras, we understand 
that it is the second star in the Lyre ; and so of 
the rest. As all who study astronomy should be 
acquainted with these signs, we insert the Greek 
alphabet for the advantage of those who have not 
studied the language: 



Alpha, . 


. a . a 


Nu, . . . 


. V 


. n 


Beta, . . 


. ]3 . b 


Xi, . . . 


. % 


. X 


Gamma, 


• r • § 


Omicron, 


. 


o short 


Delta, . 


. 6 . d 


Pi, . . . 


. 7t 


► P 


Epsilon, 


. £ . e short. 


Rho, . . 


' P 


. r 


Zeta, . . 


. S . z 


Sigma, . 


. (S 


. s 


Eta,. . . 


. 7] . e long. 


Tau, . . 


. t 


. t 


Theta, . 


. $ £ th 


Upsilon, 


. V 


. u 


Iota, . . 


. i . i 


Phi,. . . 


. 9 


. P h 


Kappa, . 


. x . k c 


Chi, . . 


• X 


. ch 


Lambda, 


. % . 1 


Psi, . . . 


. 4 


. ps 


Mu,. . . 


. /* . m 


Omega, 


. (0 


. o long. 



THi FIXED STARS. 135 



QUESTIONS. 

What have we reason to believe respecting the fixed 
stars I How many stars are distinctly visible to the naked 
eye at any one time ? How are they distinguished from the 
planets ? 

Who invented the constellations ? How did they form 
them ? Do the stars form the figure of the constellations ? 
In which instance do they most nearly form it ? What are 
some of the names of particular stars ? By whom are some 
of the constellations mentioned ? 

What is another division of the stars ? How are they ar- 
ranged ? How many stars of the first magnitude are visible 
to us ? How many of the second ? How many of the third ? 

Among what may the Milky Way be reckoned ? How 
many stars did Dr. Herschel compute in one part of it ? 

What were the dimensions of Dr. Herschel' s large tele- 
scope ? How did Sirius appear through it ? What is the 
difference in appearance between fixed stars and planets 
when seen through the telescope ? What does this prove 
respecting the fixed stars ? 

What changes are observed among the fixed stars ? 
What do we discover by examining some of them with the 
telescope ? How many such did Dr. Herschel discover ? 
What has been the result of the labours of the astronomers 
of the present time respecting them ? What are the 
periods in which some of the double stars make their revo- 
lutions ? 

What is one of the most wonderful discoveries regarding 
these stars ? What are some examples ? 

When did Hipparchus live ? What induced him to 
make a catalogue of the stars ? At what periods have new 
stars appeared ? 

What are some examples of periodical stars, and what 
the lengths of their periods ? 

How may the distance of the fixed stars be illustrated ? 



136 OUTLINES OF ASTRONOMY. 

Into what three principal classes are the constellations 
divided ? What is the meaning of the term zodiacal ? 

How many constellations are there in the zodiac ? What 
are their names ? 

How many northern constellations are there ? What are 
the principal ones ? 

How many constellations are there in the Southern 
Hemisphere ? What are the principal ones called ? 

What is the number of stars of each magnitude in the 
zodiac ? W'hat is the number of each magnitude in the 
Northern Hemisphere ? What is the number of each 
magnitude in the Southern Hemisphere ? What is the 
whole number of stars of all these magnitudes ? 

What signs do the astronomers use to designate the dif- 
ferent stars in the constellations ? How are these applied ? 
What do they use when they have gone through with the 
Greek alphabet ? When a star is mentioned as a Leonis, 
what is its rank ? What is its rank in the constellation if 
mentioned as /? Leonis ? 




Nebulse. 



(138) 



SECTION XXIV. 
NEBULAE. 

When the heavens are examined with the tele- 
scope, we find that the stars, instead of being irre- 
gularly scattered, are gathered into great clusters. 
These are called Nebula?. All the different stars 
are supposed to belong to one of these. The sun 
and all the stars which are visible to the naked 
eye, belong to the cluster called the Milky Way, 
or the Galaxy ; this cluster is somewhat flattened, 
so that when we look towards the sides we see but 
few stars, while, by looking towards the edges, we 
see so many that they appear rather like a white 
cloud than like separate stars. How great must 
this cluster of stars be, of which the solar system 
is but an atom ! Yet the number of such nebula? 
now known is three thousand. Each of these is 
far larger than the ancients supposed the whole 
universe to be. They are of various forms, and 
some of them many more times distant than others ; 
so that some appear large and bright, and others 
are almost entirely imperceptible. It will be re- 
membered that light travels about 12,000,000 of 
miles in a minute ; yet Herschel calculated that' 

(139) 



140 OUTLINES OF ASTRONOMY. 

some of the nebulae which could be seen through 
his forty feet telescope, are so distant that light 
from them could not reach our eyes in less than 
2,000,000 of years. The telescope of Lord Rosse, 
in Ireland, is much larger than that of Herschel, 
and may enable us to explore the regions of crea- 
tion still farther. These statements may appear 
incredible ; but we should measure the universe by 
observation and reason, and not by a comparison 
with the objects which daily surround us. As well 
might the insect attempt to comprehend the extent 
of the world, by comparing it with the leaf on 
which it finds a comfortable home. 

These nebulas extend over nearly the whole 
heavens ; but there are several places which appear 
completely starless to the best telescopes yet con- 
structed. One of these wonderful places, these 
openings in heaven, as Sir William Herschel calls 
them, is in the constellation Scorpio, and the other 
in the neighbouring constellation, Serpentarius. 

QUESTIONS. 

When we examine the heavens with the telescope, how 
do we find the stars placed ? What are the clusters of 
stars called ? What bodies belong to the cluster called the 
Milky Way or Galaxy ? What is the shape of this cluster ? 

How many nebulse are now known ? What are their 
forms ? How rapidly does light travel ? How long did 
Herschel suppose light required to come from some of the 



NEBULA. 



141 



most distant nebulae that could be seen through his tele- 
scope ? What telescope is still larger than that of Herschel ? 
What is the extent of the nebulae ? Are there any places 
in the heavens in which no stars are seen ? In what con- 
stellations are two such places situated ? 



SECTION XXV. 
TIME AND ITS DIVISIONS. 

Mankind have universally agreed to measure 
time by the motion of the sun. 

In civil life, the day is the interval of time which 
elapses between the rising and setting of the sun ; 
the night is the time during which he is below the 
horizon. 

The astronomical day is the interval between 
two successive noons or midnights. 

The sidereal day is the time between a star be- 
ing on the meridian and coming to it again, and is 
always the same. 

But the astronomical day is not of the same length 
as the sidereal ; for in consequence of the sun's 
motion in its orbit, the meridian which passed 
through the sun and a star one day, will the next 
day pass through the star before it passes through 
the sun. 

The sidereal day is 23 hours 56 minutes long, 
and is the time in which the whole heavens 
revolve. The astronomical day is 24 hours long. 
But as the sun does not move through the same 
space in its orbit each day, the interval be- 
tween two successive noons is not always the 

(142) 



TIME AND ITS DIVISIONS. 143 

same ; for if it be twelve o'clock by a correct 
watch when the shadow of the dial is on the twelve 
o'clock hour line, it will not be twelve o'clock by 
the same watch, when the shadow of the dial is 
again in the same position ; it may be some min- 
utes before or after, according to the season of the 
year. 

This difference between the clock and the sun 
is called the equation of time ; its amount for each 
day is put down in the common almanacks. 

Four times in the year the clock and the sun 
agree, or the equation of time is nothing. 

The return of the sun to the same equinox 
marks the length of the year, in the same manner 
as his return to the meridian marks the length of 
the day. This period is called the tropical year. 
By observations it is found that he occupies a 
longer time to return to the same star, than to the 
equinox ; that is, if the sun and a certain star were 
on the same meridian together at one equinox ; 
they will not be on the same meridian together 
when the sun passes through the same equinox 
again. Hence, as the stars do not move, this must 
be caused by the equinoxes having moved towards 
the sun. This motion is called the precession of 
the equinoxes. 

The solar year consists of 365 days 5 hours 



144 OUTLINES OF ASTRONOMY. 

48 minutes, or 365 days 6 hours nearly. The 
common civil year contains 365 days. From this it 
is clear that, if the years were always of the same 
length, an error of nearly a quarter of a day would 
be annually committed, which in process of time 
would make great confusion in the calendar. In 
four hundred years the error would amount to three 
months, and the vernal equinox would fall in June. 

In eight hundred years the summer heat would 
be at its height about Christmas-day, and the ex- 
treme rigour of winter would happen in July. 

In the time of Julius Csesar, a regulation was 
made to correct this error, on the supposition that 
every year was 365 days 6 hours long. It was 
then provided that each fourth year should have a 
day added to it, and to consist of 366 days. We 
call the year Leap-year, and the day the 29th of 
February. This calendar was called the Julian. 

But the year is not 365 days 6 hours long; it 
wants eleven minutes to complete the six hours, 
and this caused an error, which in the lapse of cen- 
turies became considerable. In 1577, the equinox 
happening on the 1 1th of March, instead of the 21 st, 
it was then determined to rectify the error ; the 
5th of October, 1582, was called the 15th, and the 
equinox restored to the 21st of March. The new 



TIME AND ITS DIVISIONS. 145 

calendar is called the New Style, and the old cal- 
endar the Old Style. 

From Pope Gregory, under whose direction this 
alteration was made, the calendar was called the 
Gregorian. 

But as the error would happen again without 
some remedy, it was determined that instead of 
calling every fourth year a leap-year, every hun- 
dreth year, for three centuries successively, should 
be common years, and the fourth hundredth year 
should be a leap-year ; and by this means, the 
error in 20,000 years would be only one day. 

Hence, 1700, 1800, 1900, are common years, 
and 2000 is a leap-year. 

The new style was not adopted in England till 
1752, when the error amounted to eleven days. 
This was remedied by calling the 2d of Septem- 
ber the 13th. 

The Russians still continue to use the Julian 
calendar. 

QUESTIONS. 

By what have mankind universally measured time ? In 
civil life, what is the day ? What the night ? What is 
an astronomical day ? What is a sidereal day ? Why are 
the astronomical and sidereal day not of the same length ? , 

What is the length of the sidereal day ? How long is 
the astronomical ? Why is the interval between two suc- 
cessive noons not always the same ? What is the equation 
of time ? How often do the clock and the sun agree ? 
13 



146 OUTLINES OF ASTRONOMY. 

What marks the length of the year ? What is the tropi- 
cal year ? What is the precession of the equinoxes? 

How long is the solar year ? How long is the civil year ? 
To what would this difference lead, if the civil year were 
always of the same length ? What would be the error in 
four hundred years ? What in eight hundred years ? 

In whose time was a regulation made to correct this 
error ? On what supposition was it founded ? What was I 
the regulation ? What is this fourth year called ? What ' 
do we call the extra day ? What is the name of this cal- 
endar ? 

How much shorter is the year, than it was supposed to 
be in the Julian calendar ? How much variation had this 
produced in 1577 ? How was the error rectified ? How 
are the two calendars distinguished ? What was the new 
calendar called ? Why was this name given to it ? 

But, as an error would still arise, what remedy was 
adopted ? What error will still occur ? 

When was the new style adopted in England ? To what 
did the error then amount ? How was it remedied ? 

What calendar do the Russians use ? 



A GLOSSARY 

OF ASTRONOMICAL TERMS 



USED IN THIS WORK. 



A'erolite, [Gr. a^p, air; and %l0o$, a stone.] A 
meteoric stone. 

Al'titude, [Lat. altitudo, height.] The height of 
a celestial body above the horizon. 

Aphe'lion, [Gr. a,-to, from, and ^to$, the sun.] 
That point of the orbit of a body that revolves 
about the sun, which is at the greatest distance 
from that luminary. 

As'teroid, [Gr. a^p, a star, and £t$o$, a species.] 
A name given by Herschei to the small planets 
between Mars and Jupiter. 

Astrol'ogy, [Gr. aa-tpov, a star, and %oyo^ a dis- 
course.] A pretended science for teaching a 
knowledge of future events by the positions of 
the stars. 

Astron'omy, [Gr. acrfpor, a star, and vopo$, a law.] 
The science which teaches what is known re- 
lating to the stars. 

Atmosphere, [Gr. atfp)£, vapour, and cr$atpa, a 

(147) 



148 



GLOSSARY. 



globe.] The air and vapors which surround 
the earth. 

Ax'is, [Lat. axis.] The straight line, real or ima- 
ginary, about which a body turns. 

Barometer, [Gr. j3apo$, weight, and petpov, a mea- 
sure.] An instrument used for measuring the 
weight of the atmosphere, and thus indicating 
changes in the weather ; and also the height of 
mountains. 

Ce'res, [Mythological, The goddess of corn.] One 
of the asteroids. 

Com'et, [Gr. xou^tv^ from xopq, hair ; because it 
was thought to have a hairy appearance.] The 
name of a class of celestial bodies which move 
about the sun, generally in very elongated orbits. 

Conjunction, [Lat. con, together, and jugo, to 
join.] The meeting of two or more heavenly 
bodies in the same degree of longitude. 

Constellation, [Lat. con, together, and Stella, a 
star.] A group of stars which form a particular 
division. 

Declination, [Lat. de, from, and clino, to bend.] 
The distance of a celestial body from the equi- 
noctial, northward or southward. 

Di'agram, [Gr. Scoypa^ua, from 8ia, through, and 
ypa<?>Q, I describe.] A figure drawn to demon- 
strate any properties. 



GLOSSARY. 149 

Di'al, [Lat. dies, a day.] An instrument used for 
determining the hour of the day by means of a 
shadow cast in the sunshine. 

Diam'eter, [Gr. $«*, through, and pstpov, a mea- 
sure.] The distance through an object, from 
one side to the other. 

Dichotomize, [Gr. hix°^^°^ divided into two equal 
parts.] To divide into pairs. 

Digit, [Lat. digitus, a finger.] The twelfth part 
of the sun or moon. 

Disk, [Lat. discus, a quoit.] The face of a celes- 
tial body as it appears to us. 

Diur'nal, [Lat. diurnus, daily.] Happening in or 
relating to a day. 

Earth, [Sax. eard, eorth, yrth.] The name of the 
third planet. 

Eclipse', [Gr. sxteirto, I faint away or disappear.] 
Literally a defect or failure ; but, in astronomy, 
the obscuration of a celestial body by an in- 
terposed body, or the shadow of an interposed 
body. 

Eclip'tic. The circle which the sun appears to de- 
scribe in his annual revolution. It is so called 
because eclipses of the sun or moon can only 
happen when the moon is in its plane. 

Ellipse', [Gr. sMs^is, a defect.] An oval figure. 

Equator, [Lat. cequatus, made equal.] The circle 
13* 



150 GLOSSARY. 

which divides the sphere of the world into two 

equal parts, and is everywhere equally distant 

from the poles. 
E'quinox, [Lat. aquus, equal, and nox, night.] 

The line at which the sun passes the equator ; 
* at which time the days and nights are equal. 
Fo'cus, [Lat. focus, a hearth.] A point where 

rays of light, or of geometrical figures concen- 
trate. 
Hemisphere, [Gr. r^tcvu half, and ff$cupa, a 

sphere.] The half of a sphere bisected by a 

plane passing through its centre. 
Her'schel. The eleventh planet, so called from 

Dr. Herschel, who discovered it. 
Horizon, [Gr. opt£W, I bound or terminate.] The 

plane which divides the visible from the invisible 

hemisphere. 
Ju'no, [Mythological, The goddess of marriage.] 

One of the asteroids. 
Ju'piter, [Mythological, The principal deity among 

the Greeks and Romans.] The ninth and largest 

of the planets. 
Latitude, [Lat. latitudo, breadth.] The distance 

of a celestial body from the ecliptic. 
Light, [Ger. licht.] That agent which makes 

objects visible. 
Lon'gitude, [Lat. longitudo, length.] The dis- 



GLOSSARY. 151 

tance of a celestial body east or west, measured 
on the equator, from some standard meridian. 

Lu'nar, [Lat. tuna, the moon.] Relating to the 
moon. 

Mars, [Mythological, The god of war.] The fourth 
planet. 

Mer'cury, [Mythological, The messenger of the 
gods.] The first planet, reckoning from the sun. 

Meri'dian, [Lat. meridies, mid-day.] A circle 
passing through the poles and the zenith of the 
spectator, and dividing the sphere into the east- 
ern and the western hemisphere. 

Me'teor, [Gr. ^fopo*, raised above the earth.] 
A transitory body or appearance in the atmo- 
sphere. 

Na'dir, [Arabic, nazeer, opposite.] The point op- 
posite to the zenith. The zenith and nadir are 
the two poles of the horizon. 

Opaque', [Lat. opacus, dark.] A body is said to 
be opaque when it intercepts the vision. 

Opposition, [Lat. oppositio, being opposite.] Ce- 
lestial bodies are in opposition when they are 
one hundred and eighty degrees apart. 

Or'bit, [Lat. orbis, a circle.] The path in which 
a celestial body makes its revolutions. 

PaMas, [Mythological, The goddess of wisdom.] 
One of the asteroids. 



152 GLOSSARY, 

Penum'bra, [Lat. pene, almost, and umbra, a sha- 
dow.] The partial obscuration which takes 
place around the shadow of one celestial body- 
when it falls on another, causing an eclipse. 

Perihelion, [Gr. rf?pt, around, and <qUo$, the sun.] 
That point in the orbit of a body revolving 
round the sun, in which it is nearest to that 
luminary. 

Plan'et, [Gr. aatsp rfka^^^, a wandering star.] 
A star that revolves about the sun. The pla- 
nets are very irregular in the change of their 
apparent situations among the fixed stars, and 
were therefore called wanderers. 

Pole, [Gr. 7t6%o$i an axis.] The poles are the two 
points, around which the stars seem to perform 
their diurnal rotation. They are the extreme 
north and the extreme south points of the sky. 

Produce', [Lat. pro and duco, to Jead forth.] In 
geometry, to draw out in length ; to extend. 

Sat'urn, [Mythological, One of the oldest of the 
deities.] The tenth of the planets. 

Side'real, [Lat. sidus, a star.] Relating to the 
stars. 

Sign, [Lat. signum, a mark or sign.] One twelfth, 
or thirty degrees of the ecliptic. They do not 
correspond with the constellations of the zodiac, 
but are measured from the point at which the 



GLOSSARY. 



153 



sun crosses the equator, at the time of the vernal 
equinox. 

Solar, [Lat. sol, the sun.] Pertaining to the sun. 

Sol'stice, [Lat. sol, the sun, and sto, to stand still.] 
The farthest point to which the sun recedes 
north or south from the equator. 

Star, [Sax. steorra; Gr. acrfpov.] In astronomy, 
those bodies which are self-luminous and be- 
yond the solar system. 

Sun, [Sax. sunna.] The central body of the solar 
system. 

Telescope, [Gr. tr^y, distant, and axortsu, I look 
at.] An optical instrument for viewing distant 
objects. 

Tide, [Sax. tidan, to happen.] The periodical 
rise and fall of the waters of the ocean, sup- 
posed to be caused by the attraction of the sun 
and moon. 

Tropics, [Gr. Sporty, the act of burning.] The 
tropics are the parallels of declination between 
which *the sun's annual path is contained. All 
that space, therefore, over which the sun shines 
perpendicularly at any time of the year, is 
within the tropics. 

Ura'nus, [Mythological, The most ancient of the 
gods.] A name by which the eleventh planet 
is frequently designated. 



154 GLOSSARY. 

Ve'nus, [Mythological, The goddess of beauty and 
love.] The second planet, 

Ves'ta, [Mythological, The goddess of the domestic 
and public hearth.] One of the asteroids, and 
the smallest of all the known planets. 

Ze'nith, [Span, zenit or cenit.] The point directly 
overhead. If a line were extended from the 
zenith through the place of the spectator, it 
would pass through the centre of the earth. 

Zo'diac, [Gr. from £«oi>, animal ; because the 
figures of animals were used in representing the 
zodiacal constellations.] The celestial region 
to which the apparent paths of the sun, moon, 
and planets are confined. 



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